The NEOM Brine Pools - The First Discovery in the Gulf of Aqaba
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The NEOM Brine Pools – The First Discovery in the Gulf of Aqaba Sam Purkis ( spurkis@rsmas.miami.edu ) University of Miami https://orcid.org/0000-0001-8646-222X Hannah Shernisky University of Miami Peter Swart University of Miami Arash Shari University of Miami Amanda Oehlert University of Miami Fabio Marchese King Abdullah University of Science and Technology Francesca Benzoni King Abdullah University of Science and Technology Giovanni Chimienti University of Bari Aldo Moro Gaëlle Duchâtellier University of Miami James Klaus University of Miami Gregor Eberli University of Miami Larry Peterson University of Miami Andrew Craig OceanX Mattie Rodrigue OceanX Jürgen Titschack MARUM Center for Marine Environmental Sciences Graham Kolodziej CIMAS, Rosenstiel School of Marine and Atmospheric Science, University of Miami Ameer Abdulla
NEOM Article Keywords: Brine pools, Gulf of Aqaba, Red Sea, Extremophiles, Tsunamites, Hyperpycnites, Seismites Posted Date: March 15th, 2022 DOI: https://doi.org/10.21203/rs.3.rs-1420638/v1 License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License Version of Record: A version of this preprint was published at Communications Earth & Environment on June 27th, 2022. See the published version at https://doi.org/10.1038/s43247-022-00482-x.
1 The NEOM Brine Pools – The First Discovery in the Gulf of
2 Aqaba
3
4 Sam J. Purkis1,2*, Hannah Shernisky1, Peter K. Swart1, Arash Sharifi1,3, Amanda Oehlert1, Fabio Marchese4,
5 Francesca Benzoni4, Giovanni Chimienti5, Gaëlle Duchâtellier1, James Klaus1, Gregor P. Eberli1, Larry
6 Peterson1, Andrew Craig6, Mattie Rodrigue6, Jürgen Titschack7, Graham Kolodziej8, and Ameer Abdulla9
7
8
1
9 CSL – Center for Carbonate Research, Department of Marine Geosciences, Rosenstiel School of Marine and
10 Atmospheric Science, University of Miami, Miami, FL 33149, U.S.A.; Email: spurkis@rsmas.miami.edu
2
11 Khaled bin Sultan Living Oceans Foundation, Annapolis, U.S.A.
3
12 Beta Analytic Inc., Isobar Science, Miami, U.S.A.
4
13 King Abdullah University of Science and Technology, Kingdom of Saudi Arabia
5
14 Department of Biology and CoNISMA LRU, University of Bari Aldo Moro, Bari, Italy
6
15 OceanX, New York, U.S.A.
16 7
MARUM ‐ Center for Marine Environmental Sciences, Bremen, Germany
8
17 CIMAS, Rosenstiel School of Marine and Atmospheric Science, University of Miami, U.S.A.
9
18 NEOM, Kingdom of Saudi Arabia
19
20
21 KEY WORDS: Brine pools, Gulf of Aqaba, Red Sea, Extremophiles, Tsunamites, Hyperpycnites, Seismites
22
23
24 1 Abstract
25 We present the NEOM Brine Pools, the first complex of brine pools discovered in the Gulf of Aqaba. The
26 discovery was made at 1,770 m water depth and consists of one large pool (10,000 m2) flanked by three
27 minor ones (
45 2 Introduction 46 Deep sea brine pools are formed by the stable accumulation of hypersaline solutions in seabed depressions. 47 Three water bodies, the Gulf of Mexico, Mediterranean, and the Red Sea have such conditions and host 48 brine pools1‐7. Even in these water bodies, brine pools are rare, with only 100 or so discoveries across all 49 three venues. The pools are also tiny compared to their host basins, ranging in size from only hundreds of 50 square meters to a few square kilometers. 51 Despite their rarity and diminutive size, brine pools present intense oases of macrofaunal and microbial 52 biodiversity in a deep‐sea benthic environment that otherwise lacks in number and variety of species8‐10. 53 Deep sea brine pools are of intense scientific interest since their pervasive anoxia, low pH, and hypersalinity 54 represent one of the most extreme habitable environments on Earth, perhaps offering clues to first life on 55 our planet, and guiding the search for life beyond it7,11,12. The significance of Red Sea brine pools has been 56 further amplified with the discovery that the extremophile microbes that inhabit them can yield bioactive 57 molecules with therapeutic potential, including anticancer properties13. 58 Of all the venues of brine pools globally, the Red Sea boasts the highest number and its population of pools 59 can be conveniently split into two categories – those situated along the deep (>1,000 m) axial trough of the 60 basin and associated with its rift spreading axis, versus those located atop the shallower (
80 81 82 Fig. 1 Location and temperatures of the Red Sea brine pools. The Red Sea pools split into two convenient 83 categories. [1] Pools in the deep (>1,000 m) axial trough associated with the rift spreading axis (red dots). [2] 84 Pools situated on the shallow coastal shelf (0‐850 m depth) of the rift (blue dots). The NEOM Brine Pools, 85 the first discovery in the Gulf of Aqaba (GofA), is neither located on the spreading axis, in the rift basin, nor 86 on the continental shelf (green dot). 87 88 3 Results 89 3.1 Physical Setting of the NEOM Brine Pools 90 The NEOM Brine Pools (Fig. 2) are located in the Gulf of Aqaba at 1,770 m depth on the bathyal plain of the 91 Aragonese Deep. This basin is conspicuous in being >800 m deeper than the average Gulf of Aqaba, and 92 comparable in depth to much of the Red Sea axial trough (Fig. 3). Major strike‐slip faults occur along both 93 sides of the Aragonese Deep19,20. The east side is confined by the Arona fault and the Aragonese fault bounds 94 to the west. Both faults are strike‐slip, though the Aragonese fault also displays a substantial dip‐slip 95 component21. The bounding fault complex of the deep is completed to the north and south by NNW‐trending 96 normal faults (Fig. 3B). This fault geometry has a large extensional component and the deep is considered a 97 true pull‐apart basin22, the only such occurrence in the Gulf of Aqaba. Seismicity is common. Earthquakes in 98 the area frequently exceed Mw=5.0, including the 1995 Mw=7.3 Nuweiba earthquake which resulted from a 99 partial rupture of the Aragonese fault23‐28. 100 Our discovery consists of one large brine pool and three minor pools within 50 m of its borders (Fig. 3C). The 101 main pool elongates parallel to the strike of the coastline, is 260 m long, 70 m wide, and covers an area of 102 10,000 m2. The minor pools are tiny by comparison, all
109 110 111 Fig. 2 Photographs of the NEOM Brine Pools. (A) Brine surface in the foreground onlaps the ‘beach’ which 112 is characterized by a rich microbial community (orange‐to‐gray in color). In (B), at a water depth of 1,770 m, 113 a Sea‐Bird CTD system is lowered through the brine‐seawater interface, a stark transition in density which is 114 especially visible via injection of biodegradable dye (C). (D) A small (30 m2) brine pool situated 50 m to the 115 west of the main pool and associated with boulders shed from the nearby foreslope. As for the main pool, 116 the microbial community also stains the beach surrounding this small pool. 117 118 119 120 Fig. 3 Bathymetric and tectonic setting of the NEOM Brine Pools in the Gulf of Aqaba. (A) The General 121 Bathymetric Chart of the Oceans (GEBCO) provides regional context to the multibeam data acquired during 122 the OceanX‐NEOM research cruise (B). The brine pool is located at the toe‐of‐slope of the Saudi coastal 123 margin in the Aragonese Deep, a pull‐apart basin and the deepest point in the Gulf of Aqaba. This basin is 124 bounded by the strike‐slip Arona and Aragonese faults (red lines) which connect via normal faults (black 125 lines). The NEOM Brine Pool situates at the junction between the coast‐parallel Arona Fault and the NNW‐ 126 trending normal fault that extends to demark the northern margin of the basin. (C) The brine‐seawater 127 interface is at 1,770 m depth and the main pool is 260 m long, 70 m wide, and covers an area of 10,000 m2. 128 Spot soundings (black dots) indicate a maximum thickness of the brine to be 6 m in the center of the pool. 129 Three minor pools, each
131 3.2 Environmental Conditions 132 The CTD measurements from 1,200 meters depth, through the brine pool interface (1,769.26 m) and 2 133 meters below (1,771.50 m) revealed that the bathyal water column above the brine pool has a stable 21.33°C 134 temperature, salinity of 40 PSU, and dissolved oxygen of 180 mol L‐1. These parameters were constant 135 down to the brine‐seawater interface (Fig. 4). Within 20 cm beneath the brine interface, salinity rose from 136 40 PSU to values higher than the limits of the conductivity probe (120 PSU). Subsequent lab analysis of the 137 brine provided actual salinity values of 160 PSU. Dissolved oxygen values fell more than 75% at the interface, 138 reaching 50 mol L‐1 by 20 cm submergence into the brine, and further falling to a minimum value of
156 gravity. The point sources were observed up to 5 m above the surface of the largest NEOM pool with a
157 frequency of one source every two‐to‐three meters along the entire 260 m stretch of the pool that abuts
158 the slope. The most substantial of these brine seepages were associated with breccia blocks (boulders)
159 which had tumbled to rest in the toe‐of‐slope (Fig. 5A and C). In some cases, it appeared that these blocks,
160 likely shed from the nearby foreslope, have excavated seabed depressions on the bathyal plain which have
161 subsequently filled with brine (Fig. 2D). The supposition that the microbially‐stained curtains are actively
162 nourished by cascading sheets of brine is supported by the abundant colonization of these areas by
163 Apachecorbula muriatica Oliver & Vestheim, 2015, a bivalve observed to solely accumulate within the mixing
164 zone between brine and normal seawater around the pools. Further, brine could be visually seen to weep
165 from the toe‐of‐slope (visible due to shimmering within the water column resultant from the halocline
166 produced by the brine) when the stained sediments were disturbed with the ROV manipulator arm. By
167 contrast, brine was not visually evident when non‐stained areas in the slope were similarly disturbed.
168 Given its high density, the possibility of brine being drawn from the pool up the slope via capillary action can
169 be discounted. The only plausible explanation for these observations is that brine seeping from the slope
170 represents one source of brine into the NEOM pool. These seeps must flow continuously, or nearly so, to
171 sustain the halophilic microbial and bivalve communities that inhabit the sediments through which the brine
172 descends into the pool below.
173 To further examine the provenance of the brine in the NEOM pool, the chemistry of the brine was compared
174 to that of the other Red Sea brines where adequate data exist (Fig. 6A) and to the expected product of
175 evaporating modern Gulf of Aqaba seawater (Fig. 6B). Cross plotting the sulfate/chloride ratio vs. the that
176 of sodium/chloride for the largest NEOM pool and its brethren reveals the NEOM pool to separate from the
177 others by virtue of its lower ratios for both sulfate/chloride (SO42‐/Cl‐) and sodium/chloride (Na+/Cl‐).
178 This graph assists in the identification of the provenance of the brine, for which there are two plausible
179 mechanisms. The first mechanism is the dissolution of the subsurface evaporitic sequences which are
180 documented to have abundantly accumulated in the Red Sea and its associated gulfs (Aqaba and Suez).
181 These evaporites are generally attributed to the basin’s Late Miocene salinity crisis which occurred early in
182 its rifting phase29‐33. The second mechanism by which the brine could have conceivably formed is from the
183 subaerial evaporation of modern seawater in coastal sabkhas – e.g., Aref and Mannaa34 – and the
184 subsequent percolation of the enriched seawater through the coastal shelf prior to accumulation at the
185 bathyal depth of the toe‐of‐slope.
186 To the first mechanism, the dissolution of Miocene evaporitic minerals, principally halite, would serve to
187 increase the Na+/Cl‐ ratio from the value of modern Gulf of Aqaba seawater (~0.76 M/M, red dot, top‐left,
188 Fig. 6A), to a ratio close to unity (demarked by broken vertical line, Fig. 6A). The fact that the population of
189 brine pools, including NEOM, plot between this pair of endmembers is therefore consistent with their brines
190 being formed by the dissolution of subsurface halite. Like all the other brines discovered in the Red Sea, the
191 NEOM pools show significant amount of bacterial sulfate reduction. The SO42‐/Cl‐ ratio of the Gulf of Aqaba
192 seawater is ~50 mM/M while the NEOM brine has the lowest SO42‐/Cl‐ ratio yet documented in the Red Sea‐
193 Gulf of Aqaba system (~2 mM/M) (Fig. 6A). In this analysis, we assumed that all sulfur measured was present
194 as sulfate, however, given the nearly anoxic conditions of the brine pool, it is likely that some portion was
195 present as H2S, and thus the SO42‐/Cl‐ ratio would be even lower than shown here.
196 A subsurface source for the NEOM brine is further upheld by a plot of its stable oxygen isotope
197 measurements versus salinity (Fig. 6B). In this plot, the curved line charts the modelled evaporation of Gulf
198 of Aqaba seawater under a humidity of 50% (annual average for Saudi Arabia). Initializing at 40 PSU and a
199 δ18O value of 2‰, seawater evaporated to the salinity of the NEOM Brine Pool (160 PSU) should have a δ18O
200 value of 16‰. The actual δ18O value of the NEOM brine, meanwhile, is only 1‰, thereby confirming that its
201 brine is not sourced from evaporated modern seawater.
Page 6 of 25
202
203
204
205 Fig. 5 Source of the brine. Microbial staining suggests the main pool is fed by brine released from the toe‐
206 of‐slope of the eastern margin of the Gulf of Aqaba. rRNA sequencing identifies this microbial community to
207 be composed of a diverse assemblage of Gamma‐ and Alpha‐protobacteria (Fig. 12). This community
208 occupies a niche defined by the presence of hypersaline brine interfacing with oxygenated normal marine
209 waters. Hence, the microbial stain is restricted to the shore of the brine pools and cases where brine
210 emanates from the seabed. (A) Shows a breccia block partially buried in hemipelagic sediment at the toe‐
211 of‐slope. Microbial stain suggests that this block has mobilized brine from the sediment which flows via
212 gravity into the pool below. (B) Captures brine weeping from the toe‐of‐slope into the main pool. (C) Shows
213 another breccia block beneath which brine is liberated into the pool. Notice abundant shrimps Plesionika sp.
214 (Pandalidae) which were observed to scavenge animals stunned or killed by the anoxic brine. In (D) and (E),
215 up to 5 m above the pool, brine weeps from the slope. (F) Outcrops from the toe‐of‐slope 1 m above the
216 pool. The stratigraphy is interleaved with a conspicuous sheeted horizon (arrowed) determined via XRD to
217 be composed of aragonite, high‐ and low‐Mg calcite with a minor contribution of orthoclase feldspar,
218 indicative of enhanced diagenesis of foreslope sediments in the presence of hypersaline waters.
219
Page 7 of 25
220
221
222 Fig. 6 Provenance of the NEOM brine. (A) Compares the chemistry of other Red Sea pools using a cross plot
223 of the ratio between sulfate and chloride (y‐axis) vs. the ratio of sodium and chloride (x‐axis). In this data
224 space, the NEOM brine is plotted along with other Red Sea pools whose chemistry has been appropriately
225 documented (data from Table S1 of Duarte et al.18 and Table 1 of Schmidt et al.6 – locations mapped in Fig.
226 1). Color of the circles denotes the salinity of the brines, note that bins are not always evenly spaced. With
227 a salinity of 40 PSU, Gulf of Aqaba seawater plots in the top‐left of this graph by virtue of its high SO42‐/Cl‐
228 but low Na+/Cl‐ ratios. Except for Afifi, a pool reported to have an exceptionally high Na+/Cl‐ ratio18, the brines
229 varyingly occupy the lower portion of the graph because they present lower SO42‐/Cl‐ ratios than normal
230 seawater and higher Na+/Cl‐ ratios. The broken vertical line denotes the Na+/Cl‐ ratio of halite, representing
231 the maximum value that fluids dissolving salt can attain. The curve in (B) charts the modelled evaporation
232 of modern Gulf of Aqaba seawater under a humidity of 50% which would deliver a δ18O value of ~16‰. The
233 measured δ18O value of the NEOM brine, by contrast, is only 1‰, suggesting that it cannot have been
234 sourced from evaporated seawater. A more plausible provenance for the brine is the dissolution of the sub‐
235 seabed Miocene evaporites (predominantly halite) that characterize this region.
236
237 3.4 Lithostratigraphy of the NEOM Brine Pools
238 Sampled via a transect of five push cores, a defining characteristic of all the pools in the NEOM complex is
239 the four concentric zones which develop around their rims (Fig. 7). The outermost zone in this series is the
240 hemipelagic mud which constitutes the seabed of the Aragonese Deep. As the brine pool is approached, the
241 second zone encountered is characterized by its rich microbial community which stains the seabed dark
242 gray. This second zone (akin to a beach in a coastal setting) is temporarily inundated by brine when even
243 minor waves are induced into the surface of the pool. The gray zone is typically 1‐2 m wide, depending on
244 local topography at the edge of the pool. Third in line, inboard of the gray zone, lies the orange ‘swash’ zone
245 which situates immediately at the brine‐seawater interface. The orange color is also induced by a dense
246 microbial community. The bivalve A. muriatica densely inhabits both the gray and orange zones. The fourth
247 zone in the sequence is located within the brine pool proper, which we define as permanently submerged
248 beneath brine.
Page 8 of 25249
250
251
252 Fig. 7 Location and photo‐scans of the four short cores and one long core retrieved from the bed of the
253 main NEOM Brine Pool. (A) Shows the western margin of this pool. Cores #1 through #4 were pushed to 50
254 cm depth sub‐seabed, whereas Core #5 was acquired with a longer barrel and pushed to 150 cm (the depth
255 of refusal). The transect of cores was set to sample the four zones which develop as the brine onlaps the
256 surrounding seabed. As developed in (B), these zones are (i) the hemipelagic slope sediments, unafflicted by
257 the brine, (ii) a beach zone which lies just outboard of the maximum extent of the brine and is colored gray
258 by microbes. Next, (iii), a ‘swash’ zone which is distinguished by its orange color, also by virtue of microbial
259 colonization, which spans the maximum reach of the brine. Finally, (iv), the brine pool proper is defined as
260 the zone permanently submerged beneath brine. (C) Shows the five cores. Note the increase in fine‐grained
261 dark intervals, indicative of reduced conditions, for the cores acquired beneath the brine. (D) Maps the
262 geometry of the largest NEOM brine pool and the location of this transect of cores on its western margin.
263
264 Radiometric dating of six horizons from the longest core (Core #5) pushed into the bed of the brine pool
265 enabled us to derive an age model spanning 1.2 kyrs of sedimentary deposition (Fig. 8). No age reversals
266 were observed in the core, suggesting that its record is both undisturbed and continuous. To determine
267 sedimentation rate, we used a Bayesian model to generate an age‐depth relationship for the core, delivering
268 an average sedimentation rate of ~0.5 mm/yr. The core presents three broad motifs of deposition (Fig. 9A‐
269 E).
270 Accounting for 30% of the core, the first depositional motif is a series of rose‐colored coarse intervals
271 comprised of angular grains spanning the sand to granule size fraction. As determined via stereomicroscopy,
272 the mineralogy of these intervals is primarily quartz and feldspar, with minor contributions of biotite,
273 chlorite, and muscovite. Maximum carbonate content in these intervals is 10% and can mostly be attributed
274 to occasional pteropods admixed amongst the siliciclastic grains. The most prominent siliciclastic interval is
275 nearly 30 cm thick and, according to the Bayesian age model, was deposited between 525 and 650 yrs. BP.
276 Albeit thinner, seven similarly composed siliciclastic intervals are recorded in the core, of which four date to
277 older than the 30 cm‐thick event. From oldest to youngest these are, a 2 cm interval dating to 1,050 yrs. BP,
Page 9 of 25278 and 1 cm intervals at 950, 780, and 700 yrs. BP. Three coarse siliciclastic intervals predate the 30 cm‐thick 279 event and situate at 500 yrs (1 cm thick), 390 yrs. BP (2 cm thick) and 50 yrs. BP (4 cm thick). 280 The second style of deposition and accounting for 55% of the core presents as stacked fining‐upward layers 281 which span clay‐to‐silt textures. These sediments are >30% calcareous, relatively rich in total organic carbon 282 (TOC, 0.4‐0.8%), and dark brown to olive in color (Fig. 9F). The layers range in thickness from
Fig. 9 Litho‐ and chemostratigraphy of the 150 cm‐long core (Core #5) from the main NEOM brine pool. (A) Clast size analysis derived from computed
tomography (CT) scanning of the core. In this histogram plot, the left border is ‐5 phi (i.e., coarse clasts) and the right border is 2 phi (fine clasts). The
color scale varies from 0% occurrence of a given phi in 1 cm intervals in blue, up to 20% by volume in yellow. Using random colors, (B) identifies the
occurrences of individual clasts. This 3‐D visualization excellently emphasizes the ten conspicuous coarse‐grained siliciclastic turbidite intervals. Of
these, the most prominent is ‘v’ which dates to ~600 yrs. BP. (C) Is a high‐resolution digital photo‐scan of the split core and (D) is the equivalent
grayscale orthoslice reconstructed from CT. (E) Is the core description capturing grainsize and lithofacies. (F) through (I) are photomicrographs which
detail the fining‐upwards arrangement of the turbidite deposits, including coarse siliciclastic intervals (G), and the clay deposit with silt streaks during
times of uninterrupted deposition (H), also interspersed with fining‐upwards turbidite deposits (I). The chemostratigraphy of the core is captured by
variation of elemental intensities measured by XRF‐scanning and presented in the form of three ratios: Rb/Zr (J), (Zr+Rb)/Sr (K), and K/Fe (L), and the
combined abundance of quartz and feldspar (M). These traces are supplemented with discrete analyses of 27 samples for carbon stable isotope (δ13C)
(N) and total organic carbon (O). 14C ages calibrated to calendar yrs. BP with 2σ error reported (P).
Page 11 of 25290 3.5 Chemostratigraphy of the NEOM Brine Pools
291 Downcore variation of elemental intensities measured by XRF‐scanning at 2‐mm intervals are presented in
292 the form of elemental ratios (Fig. 9J‐L) and the combined abundance of quartz and feldspar as determined
293 by XRD presented at 25 mm intervals (Fig. 9M). These data were accompanied by discrete analyses of 27
294 samples for carbon stable isotope (δ13C values) and TOC (Fig. 9N and O).
295 Downcore variations of Rb/Zr, (Zr+Rb)/Sr, and K/Fe indicate at least 10 intervals of elevated ratios correlated
296 with coarse‐grained siliciclastic turbidite intervals. The Rb/Sr ratios vary from 0.01 to 0.09, (Zr+Rb)/Sr ratios
297 ranged from 0.18 to 1.77, and K/Fe ratios vary from 0.19 to 1.83. Highest Rb/Zr, (Zr+Rb)/Sr, and K/Fe ratios
298 were observed in the interval from 48 to 77 cm, where the thickest coarse grained siliciclastic sand layer
299 locates. Based on XRD analysis, downcore abundances of major silicate minerals (quartz + feldspars) varied
300 from 11.4‐89.1%, with highest values observed in the interval from 48 to 77 cm (Fig. 9M), dating to
301 approximately 600 yrs. BP.
302 The TOC contents of the thick laminae of dark colored hemipelagite above and below the 48‐77 cm interval
303 range from 0.07 to 0.79% (average=0.31%, n=27). The minimum TOC values are seen in the 48‐77 cm coarse
304 siliciclastic sand interval. Except for this interval, downcore variation of TOC does not appear to be coherent
305 with the intervals of turbidite layers. The δ13C values (Fig. 9N) vary from ‐21.76‰ to ‐19.10‰, with the
306 average of ‐19.71‰ (n=27). Although the δ13C values down the core seem to be somewhat correlated with
307 TOC, the overall correlation between these two variables is insignificant (r2=0.19). Meanwhile, TOC is
308 strongly correlated with total nitrogen (TN) (r2=0.99) (Fig. 10A), indicating that the particulate organic matter
309 in the core is derived from a common source. The TOC/TN ratio ranges from 7.72 to 11.82 with the average
310 of 8.96 (n=27). The organic geochemistry results of 27 samples (from both hemipelagite and turbidite
311 intervals) are grouped based on their δ13C values and TOC/TN ratios (Fig. 10B). Here, most of the samples
312 plot within or close to the marine algae domain35, suggesting the organic carbon preserved in the
313 stratigraphy which underlays the brine pool to have solely originated in the marine realm.
314
315
316
317 Fig. 10 Organic geochemistry of the 150 cm‐long push core (Core #5) from the main NEOM brine pool. (A)
318 Total organic carbon is strongly correlated with total nitrogen (r2=0.99). (B) Plot of δ13C values versus the
319 ratio of total organic carbon to total nitrogen atop the domains of Lamb et al.35. All but one of the samples
320 associate with the domain of marine algae, suggesting the marine realm to be the primary source of organic
321 carbon preserved in the shallow stratigraphy beneath the brine pools.
322
Page 12 of 25323 3.6 Pool‐Associated Megafauna
324 The NEOM brine pools host a rich metazoan assemblage. Eels, bigeye houndshark (Lago omanensis), and
325 flat fish (likely sharpnose tonguesole cf Cynoglossus acutirostris) behaved in a manner consistent with their
326 use of the brine as a feeding strategy (Fig. 11A‐C). These predators appeared to deliberately cruise the brine
327 surface and the eel (species as yet unidentified) was observed repeatedly dipping into the brine, presumably
328 to retrieve animals which had been shocked or killed by the anoxic waters. Such behavior was unequivocal
329 for the armies of shrimp (Plesionika sp.) that amassed on topographic highs around the rim of the pool (Fig.
330 11D‐E). When illuminated by red light, we observed this shrimp species venturing as far as the mid‐pool (>30
331 m from the rim) and plucking out small organisms disabled by the brine (Fig. 11F).
332 Mirroring the zone of microbial staining around the rim of the brine pool, the black‐colored bivalve A.
333 muriatica pervasively inhabits a 20 cm‐thick zone above the brine‐seawater interface. Seemingly agnostic to
334 substrate, the density of this suspension‐feeding bivalve, first described in the Red Sea36, is equal on soft
335 sediment and atop the breccia blocks which have tumbled to straddle the brine surface (e.g., Fig. 2D; Fig.
336 5A; Fig. 11E and G). Positioning itself as such, A. muriatica is presumed to capitalize on the mixing of brine
337 with ambient seawater which delivers a niche characterized by high salinity, low oxygen, lowered pH, and
338 modestly warmer temperature, as compared to ambient. Situated as such, it might be assumed that these
339 bivalves, and, indeed, the wider metazoan assemblage which associates with the NEOM Brine Pools, are
340 nutritionally sustained by the chemosynthetic activities of the microbial community which inhabit the rim
341 of the pools and their waters. No other live metazoans were observed in the sediments collected with the
342 bivalves, which therefore seem to exclusively inhabit the microbially‐rich sediment along the edges of the
343 pools. The same distribution of this bivalve has been reported by Oliver et al.36 around the rim of the Valdivia
344 Deep brine pool which situates at a depth of 1,525 m on the axial trough of the central Red Sea.
345
Page 13 of 25346
347
348 Fig. 11 Mega‐ and macrofauna of the NEOM brine pools. In (A), an eel, possibly Ariosoma mauritianum,
349 swims over the periphery of the main pool in the vicinity of a ubiquitously present bigeye houndshark Iago
350 omanensis (B). (C) As yet unidentified flat fish, possibly a sharpnose tonguesole Cynoglossus acutirostris,
351 collected while it was encountered skimming the brine surface. (D) A common component of the pool’s
352 megafauna, the shrimp Plesionika sp. (Pandalidae), a species which congregates abundantly atop
353 topographic highs overlooking the pools (E). Illumination via red light allows the natural behavior of this
354 species to be observed (F). Here, a Plesionika sp. is encountered >30 m from the edge of the main brine pool,
355 just above the brine surface. Observed behavior consistent with scavenging ‐ this individual and others were
356 recorded retrieving metazoans shocked or killed by the brine. The bivalve Apachecorbula muriatica
357 pervasively inhabits a 20 cm‐thick zone above the brine‐seawater interface. In (G), this narrow zone falls on
358 a slope‐sourced breccia block. (H) Sampled A. muriatica in close‐up. Note abundant production of byssus
359 thread with multiple split terminations – a characteristic of this species.
360
361 3.7 Pool‐Associated Microbial Communities
362 By means of 16S rRNA sequencing, the microbial communities inhabiting the upper 20 mm of the five cores
363 transecting the eastern margin of the main NEOM brine pool were quantified (Fig. 12). The outermost core
364 in the transect (Core #1) penetrated the hemipelagic mud constituting the broader seabed of the Aragonese
365 Deep. More than 75% of the microbes in this zone adopt an aerobic metabolism, with the majority identified
366 as Gammaprotobacteria, with minor contributions from Alphaprotobacteria, and clade SO85. The second
367 core, which sampled seabed darkened in color by a rich microbial fabric (the gray zone), boasted a similar
Page 14 of 25368 community of prokaryotes. By contrast, ~75% of the microbes identified in Core #3, which penetrated the
369 orange swash zone situated immediately outboard of the brine‐seawater interface, were anaerobic,
370 adopting a broad spectrum of metabolisms spanning sulfate reduction, reductive dichlorination,
371 fermentation, and perchlorate respiration. The dominance of anaerobic prokaryotes surpassed 85% of the
372 total community sequenced in the final pair of cores (#4 and #5) which penetrated the bed of the main brine
373 pool. The abundance of sulfate reducers occupying the bed sediments of the pools squares with the
374 depleted SO42‐/Cl‐ ratio measured within the overlying brine (Fig. 6A).
375
376
377
378 Fig. 12 Lateral distribution of prokaryote communities in and around the main NEOM brine pool. 16S rRNA
379 sequencing of the top 20 mm of the five cores reveal that the microbes inhabiting the background sediment
380 (Core #1) and gray microbial beach zone (Core #2) are 75% aerobic and 25% anaerobic. Moving closer to the
381 shoreline of the pool, this distribution between aerobic and anaerobic metabolisms is reversed in the orange
382 microbial swash zone. For the samples obtained from within the brine pool (Cores #4 and #5), only 10% of
383 the sequenced microbes are aerobic with the remaining anaerobes splitting near equally into seven classes
384 of chemolithoautotrophs.
385
386 4 Discussion
387 4.1 An Oasis of Biodiversity in an Otherwise Barren Bathyal Gulf of Aqaba
388 Six weeks of submersible and ROV dives to the bathyal depths of the Gulf of Aqaba revealed a desolate
389 seabed thickly draped with mud. As is typical for bathyal and abyssal plains, species diversity is low. The
390 NEOM Brine Pools stand in stark contrast to this monotony. At the periphery of the pools, the interface
391 between normal marine waters and the anoxic brine delivers a niche in which a rich microbial community
392 develops, stratified by the preferred metabolisms of its occupants. As the brine is approached and beneath
Page 15 of 25393 it, the hypersaline anoxic environment favors extremophile prokaryotes. Among these, the metabolism of 394 sulfate‐reducing bacteria delivers brine with the lowest sulfate/chloride ratio yet documented in the Red 395 Sea‐Gulf of Aqaba system. 396 The microbial diversity of the NEOM pools is broadly representative of those identified by equivalent studies 397 in Red Sea brine pools associated with the coastal shelf17,18, as well as those pools situated on the axial 398 spreading ridge37‐43. Perhaps an esoteric ecosystem on face value, subsea hypersaline anoxic brine pools are 399 of broad interest since they arguably represent the most extreme habitable environments on Earth. 400 Indirectly or directly sustained by the chemosynthetic activities of the microbial community, a rich metazoan 401 assemblage spanning, fish, crustacea and mollusks associate with the pools. Given their high biodiversity, 402 we believe our discovery of the NEOM Brine Pools is prescient – the coastline of the Gulf of Aqaba is rapidly 403 urbanizing. This bathyal ecosystem should be afforded the same protection as the vibrant shallow‐water 404 reefs which situate 1.7 km above the pools. 405 406 4.2 Siliciclastic Intervals Beneath the Brine Pools Likely Record Tsunami 407 The long core through the bed of the main brine pool reveals a stratigraphy that partitions into three broad 408 modes of deposition. Visually prominent because of their rose color and coarse texture are the siliciclastic 409 intervals. Our core records ten such intervals in the last 1,000 years (identified as i through x in Fig. 9). 410 Whatever mechanism delivers these intervals, reoccurs with a frequency of approximately once per century. 411 Of the ten, Interval ‘v’, dating to ~600 yrs. BP, is the thickest (30 cm). 412 The angular siliciclastic clasts comprising these coarse intervals cannot have been sourced from the marine 413 slope. Shallowing from 200 m depth, submersible dives confirmed that the slope adjacent to the NEOM 414 pools is dominated by mesophotic and then photic reef communities and the calcareous detritus that they 415 yield. Deeper than 200 m, the slope is thickly draped with calcareous mud. We therefore posit that these 416 coarse siliciclastic grains are terrigenous, likely deposited into the pool instantaneously as high‐density 417 sediment gravity flows – turbidites – an interpretation supported by the upwards‐fining composition of 418 these layers and subtle sole marks at their base. 419 There are only five mechanisms capable of transporting large quantities of terrigenous material down slope 420 into the deep basin (and into the brine pool). Of these, three can be easily discounted. First, permanent 421 rivers – of which none drain into the (hyper‐arid) Gulf of Aqaba. Aeolian deposition can be discounted too. 422 The grains that comprise the siliciclastic intervals are too coarse to be transported by wind (>80% of grains 423 in these intervals are between ‐1.0 and ‐2.0 phi, Fig. 9A). Third are storm waves impacting the shoreline – 424 the narrow Gulf of Aqaba lacks the fetch to build long‐period waves exceeding 2 m in height44 – too small to 425 be a credible means of moving substantial amounts of terrigenous material into the deep basin. 426 Another possibility is that the terrigenous sediments have been carried from the coastal plain by flashfloods 427 in ephemeral rivers (wadis) triggered by episodic rainfall, as described in Katz et al.45; wherein flashfloods 428 generated underwater hyperpycnal flows. However, the size of the sediments within the coarse horizons in 429 our core exceeds what would be anticipated from flood deposition, even with such gravity flows, for several 430 reasons. First, initial flood deposits and associated hyperpycnal plume turbidity currents consist of a wide 431 range of grain sizes but are dominated by weight/volume by fine (
438 winnowing would create at these depths, and iii), if coarse flood deposits are a reflection of drought phases,
439 there is no parallel record suggesting ten independent drought phases in the past millennium.
440 The second mechanism, to our minds the most plausible, is that the coarse intervals in our core are
441 tsunamites – terrigenous deposits entrained and transported from the coastal plain by tsunami waves. The
442 coarseness and character of the sand lenses are similar to those described in offshore tsunami deposits in
443 the northernmost portion of the Gulf of Aqaba49. This second interpretation bears consideration given that
444 the calibrated 14C age of the youngest coarse interval in the core, 50 yr. BP, closely corresponds to the 1995
445 Nuweiba earthquake (Mw=7.1)23‐25,27. This earthquake occurred along the Aragonese fault within the
446 Aragonese Deep, the small pull‐apart basin in which the NEOM Brine Pools are located, generating a modest
447 tsunami28. The ~20 yr. age discrepancy between the 1995 Nuweiba earthquake and the 50 yr. BP 14C age of
448 the youngest terrigenous core interval is well within the range of error for radiometric dating, especially
449 considering the limited calibration of the 14C method for material younger than 100 yr. BP. The 1969
450 Shadwan earthquake (Mw=6.8) was also likely tsunamigenic50. Purkis et al.51 propose that tsunami are a
451 pervasive fixture in the Gulf, generated both by coseismic seabed deformation and submarine landslides.
452 Being narrowly confined, the authors stress how the Gulf of Aqaba behaves differently to an open ocean in
453 terms of tsunami propagation and impact. Only 80 km to the south of the NEOM Brine Pool, Purkis et al.51
454 report on an incipient submarine landslide in the Tiran Straits which occurred 500‐600 yr. BP, possibly
455 spawning a substantial tsunami that, computer simulation would suggest, radiated to the north, up the Gulf
456 of Aqaba, impacting the coastline inboard of the NEOM pools. The timing of this tsunami broadly
457 corresponds to the deposition of the thickest sand lens in the core (Interval ‘v’, Fig. 9). Farther to the north,
458 evidence from Eilat alludes to a major tsunami dating to 2.3 ka BP49,52 and deposits on the Egyptian coast
459 have been proposed as tsunamigenic53, though their timing is equivocal. The available evidence hence
460 supports the episodic generation of tsunami in the Gulf of Aqaba.
461
462 4.3 Fine‐grained Bouma Sequences Deposited Beneath the Brine Pools Likely Record
463 Flashfloods
464 More than half of the length of the 135 cm core through the bed of the brine pool consists of stacked fining‐
465 upward strata which we also interpret as turbidite beds, albeit less dramatic than the exceptionally coarse
466 deposits considered in the previous section. These intervals, which are finer grained and more numerous,
467 present as classic Bouma sequences, as would be deposited during waning flow as turbidity currents move
468 downslope and out over the brine pool. These sequences span thicknesses of mm to cm, with a base of very‐
469 fine to fine‐siliciclastic sands, fining upwards to silt or clay (Fig. 9E). We consider these fine‐grained fining‐
470 upward turbidites to be consistent with the hyperpycnites described by Katz et al.45 farther north in the Gulf
471 of Aqaba, deposited by muddy submarine hyperpycnal flows generated by flashfloods. These flows, filmed
472 underwater by Katz et al.45, originate from the terrestrial discharge of sediment‐laden flood waters from
473 wadis during short‐lived rain events (hours). Flashfloods in the Gulf of Aqaba may occur at a frequency of
474 between several times in a year to once every several decades45,54, corroborating the large number of
475 stacked fine‐grained turbidites in the core. There are at least 50 such fining‐upward sequences in the core,
476 corresponding to a reoccurrence frequency of at least four times per century.
477
478 4.4 The NEOM Brine Pools Likely Also Record Seismicity
479 Situating along the Arona Fault and only 5 km away from the Aragonese Fault (which slips often – as attested
480 to by the 1995 Nuweiba earthquake and associated aftershocks), the NEOM Brine Pools likely experience
481 frequent seismicity. At three intervals, contorted beds were recognized in the core (Fig. 9C and E), consistent
482 with seismic disturbance. Bosworth et al.21 document the epicenters of at least 15 earthquakes to situate
483 within 50 km of the pool between 1960 and 2016, of which two exceeded Mw=5.0. In 1993 alone,
Page 17 of 25484 Hofstetter26 meanwhile identified an earthquake swarm of over 420 events to have occurred within and
485 nearby the Aragonese Deep. Induced by such seismicity, rates of footwall uplift for the Saudi coastal terrace
486 adjacent to the pool exceed 0.010 cm/yr when averaged over the last 125 ka21. Even minor shaking should
487 be expected to induce down‐slope movement of sediments, particularly given that the foreslope adjacent
488 to the pool is both the steepest (>35°) and deepest (>1,700 m) in the entire Gulf of Aqaba (Fig. 3B).
489 Configured as such, the brine pool is likely the frequent recipient of seismically‐induced turbidite deposits –
490 seismites. Separating hyperpycnites (flashflood deposits) from seismites lies beyond the scope of this paper,
491 but will be tackled by comparing the historical earthquake record of the Gulf with the dated deposits in our
492 core.
493
494 4.5 The NEOM Brine Pools are Natural Sediment Traps that Exquisitely Record Coastal
495 Processes
496 Although event beds from tsunami, flashfloods, and seismicity would be deposited abundantly at the toe‐
497 of‐slope in the Aragonese Deep, the brine pools offer exceptional conditions conducive to their intricate
498 preservation. First, because of the anoxic brine, megafauna is eliminated from the pools and bioturbation is
499 entirely absent. Second, the high density of the brine effectively shields the bed from deep‐water currents
500 (contourites) which would otherwise winnow or erode the bathyal seabed.
501 The third exceptional property of the pools is the way that descending turbidites will interact with the brine
502 prior to depositing on the bed of a pool. As can be visualized via the injection of dye (Fig. 2C), the seawater‐
503 brine interface delivers a stark density gradient. Indeed, so dense is the brine that we could land our 10,000
504 lb ROV on its surface to choreograph deployment of the CTD (Fig. 2B). Upon reaching the surface of the
505 brine, this density gradient would be sufficient to temporarily arrest a turbidite, the material from which
506 would then settle through the brine to deposit on the seabed, with the dense grains settling swiftest, and
507 so on, until the finest grains settle last, capping the fining‐upward interval. Since the energy of the
508 descending turbidite would not immediately reach the pool’s bed (it would instead be absorbed by the
509 seawater‐brine interface), the plume would not scour, disturb, or destroy the sediments deposited by
510 previous events. In this way, even layers with mm‐scale Bouma sequences are delicately preserved in the
511 base of the pool, potentially delivering an exquisite long‐term record of tsunami, flashfloods, and seismicity
512 in the area. We hence believe that the brine pools have a remarkable property that might commend them
513 to both paleoclimatologists and to seismologists.
514
515 5 Conclusions
516 Analysis and characterization of the NEOM Brine Pools has resulted in the establishment of the first brine
517 pool in the greater Red Sea system to be located sufficiently close to the coastline to act as a sediment
518 deposition trap for slope processes. Situated as such, the NEOM pools have unique potential to archive
519 historical tsunami, flashfloods, and seismicity in the Gulf of Aqaba on millennial timescales. By ascertaining
520 that brine pools form outside the rifting portion of the Red Sea, in the Gulf of Aqaba, we hope to have
521 expanded the notion of how brine pools are formed and where. For the first time, we have also emphasized
522 the value of brine pools as sediment archives, by revealing a multi‐phase, multi‐event sequence of
523 sedimentary deposits preserved beneath the NEOM pools.
524
Page 18 of 25525 6 Materials & Methods
526 6.1 Swath Multibeam, Sonar, and ROV
527 Building forward from mapping published by Ribot et al.20, the 2020 OceanX ‘Deep Blue’ Expedition acquired
528 a comprehensive geophysical dataset spanning the northern Red Sea and Gulf of Aqaba. The expedition was
529 conducted aboard OceanXplorer, an 87 m research vessel furnished with state‐of‐the‐art tools for seabed
530 mapping, including a pair of custom Triton submersibles with a diving depth of 1,000 m, plus a 6,000 m‐
531 rated Argus Mariner XL remotely operated vehicle (ROV). These assets were used exhaustively throughout
532 the six‐week expedition to collect seabed and water samples, cores, and geophysical data encompassing
533 bathymetry, sub‐bottom, and ADCP (acoustic Doppler current profiler) lines. Key to supporting this study is
534 the swath multibeam data acquired using hull‐mounted Kongsberg EM712 and EM304 systems operating in
535 the 70‐100 kHz range calibrated with CTD (conductivity, temperature, depth) and expendable
536 bathythermograph (XBT) profiles. The beam density of these systems was configured to deliver a minimum
537 across‐track resolution of 10 m. Upon discovery of the pools, we conducted a series of ROV dives to map
538 their geometry, obtain samples, and acquire high‐resolution video and still footage (Fig. 2). The pools lacked
539 a multibeam signature but, by virtue of their dampened backscatter as compared to the surrounding seabed,
540 were detected by the ROV‐mounted 450‐900 kHz imaging sonar, aiding navigation and mapping.
541
542 6.2 Sampling the Brine
543 We lowered a Sea‐Bird 911+ CTD system equipped with a rosette of 12× 12L Niskin bottles into the main
544 brine pool at 28° 47.31587' N, 34° 47.70937' E. For each of multiple casts, the rosette was triggered to obtain
545 water samples 500 m and 5 m above the seawater‐brine interface, at the interface, and below it (Fig. 2B).
546 Simultaneously, we collected vertical profiles of salinity, dissolved oxygen, and temperature (Fig. 4). Spot
547 soundings confirmed that the center of the pool was 6 m deep, allowing for the Sea‐Bird to be completely
548 submerged in the brine. So dense was the brine surface, that the ROV could ‘land’ on the brine surface and
549 the CTD casts were choreographed in real‐time from the vehicle’s high‐definition video stream.
550 The salinity of these water samples was determined using a refractometer, the oxygen isotopes (δ18O) using
551 a Picarro cavity ring‐down spectrometer, and chloride using titration with 0.1 silver nitrate using potassium
552 permanganate as an end‐point indicator and calibrated using IAPSO (International Association for the
553 Physical Sciences of the Oceans) standard seawater. Total alkalinity and initial pH were measured following
554 the workflows published by Gieskes et al.55 which employ a potentiometric titration of acid handled on‐line
555 by a computer interfaced with a pH meter and a Metrohm autoburette. Elemental concentrations were
556 measured on an Agilent 8900 ICP‐MS/MS after samples were diluted by factors proportional to their salinity.
557 SPEX CertiPrep multielement solutions, IAPSO, and CASS‐6 were used as certified reference materials.
558
559 6.3 Cores, CT, and a Radiocarbon Age Model for the Brine Pool
560 A transect of five cores was acquired across the western rim of the main brine pool to sample the four
561 concentric microbially‐stained zones which develop around its rim (Fig. 7). Using the ROV’s hydraulic
562 manipulator arm, each of the four zones was sampled with a 50 cm push core (Cores #1 through 4), plus one
563 additional long core (150 cm) was pushed into the sediments accumulating beneath the brine (Core #5).
564 The long core was scanned via computed tomography (CT) to reconstruct its three‐dimensional lithology.
565 The CT data were processed with the ZIB edition of the Amira software [v. 2021.0356] to yield maps of
566 grainsize, clast distribution, and orthoslices. A full description of this processing stream is given in
567 Supplementary Materials, section S1.
Page 19 of 25568 Following CT scanning, pore waters were extracted from the cores via drilling through their liners at 25 mm
569 increments and inserting Rhizon samplers which filter samples to 0.22 µm57. The recovered pore volumes
570 ranged in volume from 5‐15 cm3, and samples were immediately analyzed for alkalinity, salinity, and chloride
571 concentration. Prior to elemental analysis, 0.5 µl of 5% UT HNO3 was added to aliquots of porewater, brine,
572 and seawater samples, and stored at 4 °C. With their pore waters extracted, the cores were sliced to enable
573 sedimentological description and physical sampling for geochemistry, mineralogy, genomic analyses, and
574 radiometric dating.
575 As detailed in Supplementary Materials, section S2, radiocarbon (14C) dating was conducted on the long core
576 acquired from the center of the brine pool. Six samples were extracted at prominent stratigraphic horizons
577 (Fig. 8) and submitted to Beta Analytic Inc. for dating. These ages and their associated uncertainties were
578 calibrated against the current marine calibration curve [Marine2058] using the Calib8.2 online platform59. To
579 calibrate the 14C dates against Marine20, a local value for the regional‐marine radiocarbon reservoir age
580 correction (ΔR) is required. The ΔR for the Gulf of Aqaba was calculated following Reimer and Reimer60 using
581 the average U‐Th age for a mesophotic coral sample collected in the southern Gulf51. At the 95% confidence
582 range, the local ΔR of ‐44±74 was used to account for the marine radiocarbon reservoir age corrections. The
583 calibrated 14C ages are reported as calibrated year before present (cal. yr. BP).
584 The original length of the long core, 135 cm, compressed to 85 cm during acquisition. Before attempting to
585 construct the age model, the length of the core and the position of sub‐samples for radiocarbon dating was
586 adjusted to the original length of the core. An age‐depth model was constructed based on a Bayesian
587 approach using the “Bchron” package61,62. We used the ‘accumulation’ output field in Bchron to create
588 sedimentation rates for the sediment core (Fig. 8), defined as the thickness of sediment accumulated during
589 a given unit of time in mm/yr. Several layers deemed to have been deposited by high‐density grain flows
590 with different thicknesses, ranging from 1 to 300 mm, were identified along the core, which are indicative
591 of rapid deposition during a very short time. Deposition of these layers were not incorporated in our
592 calculation of sedimentation rates. As a result, the presented sedimentation rates (Fig. 8) should be
593 considered as overall rates.
594
595 6.4 X‐Ray Fluorescence and X‐Ray Diffraction Scanning
596 X‐ray fluorescence (XRF) data were acquired using the Avaatech XRF core scanner at the University of Miami.
597 Full procedures and scan settings are provided in Supplementary Materials, section S3. We capitalize on the
598 XRF results to diagnose the provenance of the stratigraphic horizons recognized in the long core through the
599 brine pool (Fig. 9J‐L). Heavy minerals in siliciclastic sediments are enriched in zirconium (Zr)63. Consequently,
600 in marine cores Zr abundance can be used to proxy coarse siliciclastic intervals in a sedimentary profile64. In
601 a similar vein, Rubidium (Rb) often appears in potassium silicates, such as K‐feldspar, illite, and muscovite,
602 and may be used as a measure of amounts of clay material or coarse‐grain K‐feldspar present in the
603 sedimentary record63,65. Iron (Fe) and potassium (K) are elements related to aluminosilicate minerals, such
604 as K‐feldspar and biotite, and associate with the input of terrigenous sediment into marine basins66‐72.
605 Whereas, strontium (Sr) can be associated with feldspar and biotite66, the concentrations are low, and this
606 element primarily associates with the carbonate fraction of sedimentary successions, particularly calcite and
607 aragonite, owing to substitution of Sr for Ca within the carbonate lattice.
608 To determine mineralogy, X‐Ray Diffraction (XRD) was conducted on powdered samples extracted at 25 mm
609 intervals down the cores. Full details of this analysis are provided in Supplementary Materials, section S4.
610 Results from XRD facilitated calculation of the percentage of feldspar and quartz (Fig. 9M), thereby
611 identifying the coarsest intervals in the core as siliciclastic.
612
Page 20 of 25613 6.5 Stable Isotope Geochemistry
614 As treated in detail in Supplementary Materials, section S5, stable isotopes analyses of the carbon and
615 nitrogen in the organic matter were also conducted at 25 mm intervals down the cores (Fig. 9N), analysis
616 which also yielded the percentages of organic carbon and nitrogen (Fig. 9O). These data were used to
617 identify the source of organic matter in the sediments which have accumulated in the brine pool.
618
619 6.6 16S rRNA Sequencing of Core Tops
620 The upper 20 mm of each of the five cores was sampled for their microbial communities using 16S rRNA
621 sequencing. For the sake of brevity, full detail on the extraction of genomic DNA is relegated to
622 Supplementary Materials, section S6. The fidelity of the sequencing was sufficient to identify seven classes
623 of prokaryotes whose per‐core relative abundances were tallied to reveal gradients in the prokaryotic
624 community in and around the brine pool.
625
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