Neogene sapropels in the Mediterranean: a review
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ELSEVIER Marine Geology 153 (1999) 11–28
Neogene sapropels in the Mediterranean: a review
Adrian Cramp Ł , Gerard O’Sullivan
Department of Earth Sciences, University of Wales, Cardiff, CF1 3YE, UK
Received 15 May 1997
Abstract
For 50 years the existence of sapropels (organic-carbon-rich sediments) deposited within Plio–Pleistocene sediments
of the Mediterranean Sea has been known. Initially, research concentrated on material recovered in relatively short
gravity=piston cores taken from the eastern basins where sequences were found to be well developed=preserved and had
extensive spatial coverage. In the main, previous studies concentrated upon establishing a workable stratigraphy, spatial
correlation of individual layers and determining the probable depositional mechanisms. However, despite a plethora of
research papers, some issues still remain unresolved. This is in part due to a lack of agreement between investigators;
sampling and analytical short comings, restricted sample size and the fact that, in many instances, like was not being
compared with like. Recently, the limit of sapropels in the western basin has been further extended. As a result, the
palaeoceanographic=palaeoclimate models which had previously been developed for deposition of sapropels in the eastern
basin have been modified. Most recently, strong links have been established between astronomical cyclicity and sapropel
formation. This review paper provides a summary of sapropel research to date, and ongoing sapropel research in the
Mediterranean, some of which appears in this thematic issue of Marine Geology. It is fitting that this thematic issue of
Marine Geology be dedicated to the memory of Colette Vergnaud-Grazzini and Rob Kidd who in many ways helped to
initiate the resurgence in sapropel studies in the 1970s in the Mediterranean —perhaps in 50 more years we will know all
of the answers! 1999 Elsevier Science B.V. All rights reserved.
Keywords: Neogene; sapropels; astronomical cyclicity; climate amelioration; Mediterranean
1. Introduction deposited within semi-enclosed marine basins. The
latter form the basis of this contribution.
Despite an ever increasing literature, the depo- To date, the majority of sapropel studies have at-
sitional processes and climatic amelioration which tempted to propose prerequisites required to preserve
have resulted in the deposition and=or preservation enhanced levels of carbon at or near the seafloor. In
of organic-carbon-rich sediments through geological the main, these studies have used a wide variety of
time remain, in many instances, unconvincingly re- analytical techniques on a wide range of samples to
solved. Organic-carbon-rich deposits are relatively determine whether or not depleted oxygen and=or
common within marine sedimentary sequences, the primary productivity are the key factors determining
most notable being the Black Shales of the Creta- elevated organic-carbon preservation. Whilst this ap-
ceous, and the much cited Plio–Pleistocene sapropels proach has produced a plethora of papers, many key
areas we believe have, and to a certain extent, still
Ł Corresponding author. E-mail: cramp@cardiff.ac.uk remain unresolved.
0025-3227/99/$ – see front matter c 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 5 - 3 2 2 7 ( 9 8 ) 0 0 0 9 2 - 912 A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28
In recent years, the advances made in coring tech- virtually continuous Holocene–Pliocene sequences
niques in association with the rapid development recovered across entire basins, has it been possible
of analytical systems and predictive modelling tech- to prove the dramatic lack of spatial and tempo-
niques, has meant that the value of sapropel-related ral continuity of sapropels (cf. Emeis et al., 1996).
studies is rapidly being re-addressed. Data now be- However, we are now in a better position to produce
ing generated from Neogene sequences provide high- meaningful correlations on both individual horizons
resolution information with regard to the history of and between basins. In addition, continuous long
climate change and water mass circulation in semi- core recovery enables more realistic analogies to be
enclosed marine basins (for example, the Mediter- drawn between the marine record, and comparable
ranean and Sea of Japan) which appear to be sensitive land sequences (Emeis et al., 1996).
to climatic amelioration. In turn, this high-resolution In recent years, strong links have been estab-
palaeoceanographic and palaeoclimatic information lished between sedimentary cycles and astronomical
will enable more accurate forecasts to be made in ar- time scales. Data sets acquired from both marine
eas of high population where subtle changes in climate and land-based sediments have provided excellent
and water circulation appear to have tipped a sensitive accuracy and resolution, especially over the last 15
oceanographic balance in the recent geological past. million years (Shackleton et al., 1995). These tech-
The current state of understanding is still far from niques have been very successfully applied to land
complete. This in part is due to the fact that previous sections in the Mediterranean which contain sapro-
studies have not, in all cases, compared like with pel sequences (Hilgen, 1991a; Lourens et al., 1996;
like. There appears to have been confusion in the Hilgen et al., 1997). Continuing work is extending
past when attempting to compare marine sapropel these innovative land-based studies to the relatively
sequences in both time and space since, until re- long continuous marine sequences recovered in 1995
cently, it was not realised that there was considerable by the Ocean Drilling Program (Lourens, 1998).
temporal and spatial variation in the occurrence and This contribution provides a brief history of mul-
preservation of sapropel sequences. For example, the tidisciplinary sapropel research carried out to date in
most widely cited stratigraphic correlation of eastern the Mediterranean.
Mediterranean sapropels provided 25 years ago by
McCoy (1974) has undoubtedly been misinterpreted 2. Sapropel nomenclature
when incomplete and=or diagenetically altered se-
quences have been used, and misleading correlations In describing organic-carbon-rich sediments, the
and analyses have resulted. Despite the fact that term sapropel, originating from the Greek sapros
initial deposition of sapropels, in most instances, ap- (rotten) and pelos (soil), was widely cited follow-
pears to be a basin-wide phenomenon (though not ing the work of the German chemist Wasmund who
all synchronous cf. Strohle and Kron, 1997), se- qualitatively described the composition of sapropel,
quences recovered do not always contain complete gyttja (a freshwater decomposition of plant material)
chronological=stratigraphical successions. and bitumen from lacustrine sediments (Wasmund,
The lack of complete sequences appears, in many 1930). Eight years later, Bradley hypothesised that
cases, to be the result of redeposition and=or post- organic-rich sediments could have been deposited in
depositional geochemical alteration processes (redox the Mediterranean as a result of fluctuations in Pleis-
in the main) which either visually remove completely tocene sea levels (Bradley, 1938). Sapropels were
or alter the nature and composition of individual or first recovered from the Mediterranean during the
groups of sapropels in some geographical areas (cf. Swedish Expedition to the area in 1947 (Kullen-
Higgs et al., 1994). Only very recently, with the use berg, 1952). Olausson (1961) later introduced the
of long and continuous multicoring (for example, the term sapropelitic rather than sapropel, to describe a
ODP Advanced Piston Coring system —APC) used suite of late Quaternary organic-carbon-rich marine
in association with continuous multi-sensor data sediments recovered from the eastern Mediterranean.
(p-wave, gamma-ray and spectral information) and The term remained mostly qualitative until 1978
high-resolution geochemical analyses carried out on when Kidd et al. first proposed a quantitative defini-A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28 13
tion of sapropels. The widely adopted nomenclature and sapropelic-containing cores have been recovered
originated from sequences recovered during Leg 42A from the area. In addition, five drilling legs have oc-
of the Deep Sea Drilling Program (DSDP —the first cupied sites within the semi-enclosed basin; Legs 13
successful academic drilling of Holocene–Miocene and 42A of the Deep Sea Drilling Project (DSDP) and
sequences in the Mediterranean) defines sapropels Legs 107, 160 and 161 of the Ocean Drilling Program
as being: “A discrete layer, greater than 1 cm in (ODP). These drilling legs have recovered sapropels
thickness, set in open marine pelagic sediments con- from a total of 28 sites (Fig. 4). To date, the sapro-
taining greater than 2% organic carbon (Corg ) by pels recovered, in the main, date back to the Early
weight” (Fig. 1). Kidd et al. (1978) further defined Pliocene, though the earliest reported occurrence is
sapropelic sequences (in the past included within the from the Middle Miocene (Kidd et al., 1978) and are
entire suite of organic-carbon-rich sediments) as be- intercalated within and below evaporite sequences.
ing similar marine deposits containing “between 0.5 The highest levels of organic matter preservation
and 2% Corg by weight” (Figs. 2 and 3). within sapropels appear to be limited to the area
Whilst the Kidd and co-investigators nomenclature to the east of the Sicilian Sill. Until recently, only
provided a useful and, at the time, usable starting point two geographical areas had yielded cores containing
for many workers, with advancing techniques, espe- sapropel sequences in the western basin: the Balearic
cially within the field of palaeoceanography, it became Rise (Ryan, 1972) and the Tyrrhenian Sea (Kidd et
of limited use in that it failed to take into account depo- al., 1978). In 1986, ODP Leg 107 recovered more
sitional conditions and=or post-depositional processes complete Plio–Pleistocene sapropels sequences from
which may have significantly reduced=enhanced the the Tyrrhenian Sea (Emeis et al., 1990; Emeis et
carbon content of the sediments. The recognition or al., 1991). ODP drilling across the Mediterranean in
lack thereof of carbon preservation has become in- 1995 (Legs 160 and 161) recovered the most com-
creasingly important to many workers. plete sapropel sequences to date (Emeis et al., 1996;
The early 1990s saw a resurgence in sapropel re- Comas et al., 1996; Fig. 5), and has further proven
search and in 1991 Hilgen (1991a) proposed a less their existence and geographical distribution in the
restrictive definition in describing the term sapropel. western basin (in the western basin sapropels and
Hilgen’s definition states that sapropels are simply sapropelic layers were termed organic-rich layers;
“brownish, often laminated interbeds.” This defi- ORLs —sensu Comas et al., 1996).
nition in many cases is less restrictive than that To date, sapropels (sensu Kidd et al., 1978) have
originally proposed by Kidd et al. (1978). However, not been recovered from areas which have a pre-
Hilgen’s definition cannot be applied to many Plio– sent-day water depth of less than 500 m, whereas
Pleistocene sapropels; in fact, the majority of Plio– sapropelic sediments have been recovered from wa-
Pleistocene sapropels recovered during the recent ter depths as shallow as 125 m (Cramp et al., 1988;
ODP legs displayed no lamination, the most notable Perissoratis and Piper, 1992).
exception being the widely reported S5 (12) where Tectonic activity and bathymetric configuration
it appears that in some instances rapid burial in have resulted in a high degree of temporal and spa-
small anoxic basins, in some instances not undersat- tial variability in the distribution of sediments within
urated in silica, may have preserved seasonal-scale the Mediterranean. This is particularly true for sapro-
mass siliceous (rhizosolenid-laminae) sedimentation pels and sapropelic sediments where a large degree
(Pearce et al., 1998). As sapropel research continues of spatial and temporal variability exists. For ex-
post-ODP cruises, it may well be that the sapropel ample, within depositional basins at the distal end
nomenclature issue will have to be re-addressed. of sediment transport pathways extended sapropel
sequences, some in excess of 4-m-thick, have been
3. Marine sapropel sequences recovered (in the main thought to be due to re-sed-
imentation through gravitational processes). By way
Sapropels were first described in gravity cores of comparison, undisturbed sapropels and=or ORLs
recovered from the Mediterranean in 1947 (Kullen- recovered from bathymetric highs vary in thickness
berg, 1952). Since then, in excess of 1500 sapropel- between approximately 2 and 25 cm (Figs. 1–3).14 A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28 Fig. 1. Sapropels (dark-coloured intervals, 4.12–7.10% Corg ) recovered from a ridge site to the north of the Eratosthenes Seamount in the eastern Mediterranean. The interval (75.8–85.3 m below seafloor) contains well developed middle to Late Pliocene sapropels intercalated with oxygenated sediments (light-coloured). Bioturbation, mainly Chondrites and Planolites, is evident in most sapropels, and is usually indicative of reduced oxygen conditions.
A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28 15 Fig. 2. Early Pleistocene sapropelic layers (dark-coloured intervals, 1.3% Corg ) recovered from the Gela Basin on the Sicilian Sill. The interval (162.5–170.5 m below seafloor) contains two sapropelic layers (section 3, 50–80 cm and section 5, 45–115 cm). Both sapropelic layers have been extensively burrowed by Chondrites and Zoophycos.
16 A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28 Fig. 3. Pleistocene sapropelic layers (organic-rich layers, ORLs, 1.52–1.71% Corg ) recovered from the Menorca Rise on the South Balearic Margin in the western Mediterranean. Some bioturbation (Chondrites) is evident in the sapropelic layer in section 2, 112–120 cm.
A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28 17
Fig. 4. Mediterranean basin showing the location of Deep Sea Drilling Project (DSDP) sites (circles), and Ocean Drilling Program (ODP)
sites (dots).
4. Correlation and stratigraphy through to the oldest recovered (at that time) sapro-
pel S12 (dated at approximately 400,000 BP).
Until relatively recently, with the introduction and Sapropel S1 has been recorded within the sedi-
refinement of the astronomical time scale in relation ments recovered in most gravity and piston cores col-
to sedimentation, attempting to provide a workable lected from extensive areas of the eastern basin. The
correlation=stratigraphy of marine sapropel-contain- horizon correlates with an interval of oxygen isotope
ing sequences was solely based upon traditional Stage 1, and is up to 20-cm-thick (Stanley and Mal-
dating techniques. Despite the fact that Kidd et al. donado, 1977). S1 has an organic-carbon (Corg ) con-
(1978) had reported on the possible presence of tent of approximately 2% (Cita et al., 1982). Sapropel
sapropel sequences of Miocene age, much research S2 is generally less than 5-cm-thick, and is frequently
concentrated on the sequences which had been de- missing and=or is not visible in the sedimentary se-
posited in the last 400,000 years (Table 1). quences. Deposition of S2 occurred during a phase
From this work it appeared that most periods of oxygen isotope Stage 3. Sapropels S3 , S4 and S5
of sapropel formation corresponded with warming are closely related in time, all being deposited dur-
phases of climate and, as such, many workers used ing oxygen isotope Stage 5. Sapropel S5 , in many
the oxygen isotope record as a guide for correlation cases is=was the most visually striking of the origi-
purposes. nal limited sequence, usually being devoid of biotur-
The first substantial contribution in attempting bation, commonly containing micrometre-scale lami-
to correlate sapropels was presented by McCoy in nae. This sapropel is dominated by a warm water car-
1974. Using sedimentological and micropalaeonto- bonate microfossil assemblage (Globigerinoides ru-
logical data generated for sapropels recovered from ber), and it has been widely reported that the deposi-
gravity and piston cores, McCoy provided a corre- tion of this sapropel coincided with oxygen isotope
lation which assigned the most recently deposited sub-Stage 5e (approximately 125,000 BP). Sapro-
sapropel as S1 (dated at between 6 and 12,000 BP) pels S3 and S4 are approximately 10–15-cm-thick,18 A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28 Fig. 5. Schematic time scale diagram illustrating the location of drill sites across the Mediterranean. The main periods of sapropel= sapropelic deposition are represented by the dark shading on the core logs.
A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28 19
Table 1
Sapropel sequences deposited in the last 400 ka
Sapropel Age (ka) Author code Oxygen isotope stage
14 C
McCoy–Hilgen (i-cycle) astronomical
S1 2 6.3–8.3 1, 11, 12 1
6.4–9.2 2, 11, 12 1
6.6–8.2 3, 11, 12 1
8–9 4, 11, 12 1
7.9–11.8 5, 11, 12 1
S2 6 23–25 3, 11, 12 3
52 6, 11, 12 3
S3 8 38–40 4, 11, 12 5a
81–78 6, 11, 12 5a
80 7, 11, 12 5a
84–80 8, 11, 12 5a
S4 10 100–98 6, 11, 12 5c
100 7, 11, 12 5c
S5 12 125 6, 8, 11, 12 5e
7, 9, 11, 12 5e
S6 16 7, 10, 11, 12 7
S7 18 195 7, 11, 12 7a
S8 20 217 7, 10, 11, 12 7d
S9 22 240 10, 11, 12 7e
S10 30 331 10, 11, 12 9
S11 38 407 10, 11, 12 11
S12 46 461 10, 11, 12 11
1 D Jorissen et al. (1993); 2 D Perissoratis and Piper (1992); 3 D Maldonado and Stanley (1978); 4 D Olausson (1961); 5 D
Rossignol-Strick et al. (1982); 6 D Muerdter et al. (1984); 7 D Oggioni and Zandini (1987); 8 D Parisi (1987); 9 D Cita et al. (1982);
10 D Cita et al. (1977); 11 D Hilgen (1991a,b); 12 D Lourens et al. (1996).
and have Corg contents of approximately 2%, whereas Until the early 1990s, the nomenclature and cor-
sapropel S5 is up to 22-cm-thick, and has a Corg content relation proposed by McCoy was adopted by most
of approximately 7% (Oggioni and Zandini, 1987). workers; however, with innovative work being car-
Sapropels S6 (Stage 6) and sapropels S7 and S8 ried out on the refinement of the astronomical time
(Stage 7) are closely related in time. The absence of scale signal as reflected in marine sediments, it be-
warm water foraminiferal faunas such as G. ruber and came increasingly apparent that the McCoy scheme
the presence of a cold climate faunal assemblage indi- when applied to Mediterranean sapropels was inac-
cates that sapropel S6 is anomalous to other sapropels curate and inflexible. In the late 1980s and early
in that it was deposited during a cooler period (Cita 1990s work carried out by a group based in Utrecht
et al., 1982). Corg levels of up to 5% have been re- provided an alternate to the McCoy nomenclature.
ported for both sapropels S6 and S7 . Sapropel S7 has The Utrecht group based their stratigraphy upon the
a faunal assemblage indicative of a warm climate, astronomical time scale. Initially, Hilgen (1991a) in-
whereas sapropel S8 has a faunal assemblage similar troduced an astronomical calibration of Gauss to
to sapropel S6 suggesting a cooler climate and perhaps Matuyama sapropels in the Mediterranean area (fig.
a low-salinity surface layer (Cita et al., 1977). 6 of Hilgen, 1991a) and later extended this cali-
Sapropels S9 to S12 have only been recorded in bration to the Miocene=Pliocene boundary (Hilgen,
the longest and most complete piston cores and are 1991b). Subsequently, Lourens et al. (1996) refined
usually thin in extent (1–2-cm-thick). These sapro- and evaluated these time scales. These studies ap-
pels in the main are extensively bioturbated (Cita et pear to have been very successful in explaining the
al., 1982). timing and occurrence of individual sapropels and20 A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28 sapropel sequences or ‘bundles’ (see later). As a 5.2. Anoxia: the cause or a symptom of sapropel result of the work carried out by Hilgen, Lourens formation? and others, the ages of individual sapropels was as- signed based on a time-lag of 3 ka between sapropel In water, conditions can be quantified as oxic if formation and its correlative precession minimum dissolved oxygen concentrations >5‰, while con- (insolation index). A new nomenclature was pro- centrations
A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28 21
of climatic warming (Ryan, 1972). At such times, in- the Mediterranean via increased Nile discharge and
creasing temperatures would cause the continental increased activity in Mediterranean atmospheric de-
ice caps to the north of the Mediterranean region to pressions. These depressions would interact to pro-
melt resulting in an influx of glacial meltwater into duce a reduction, but not a complete reversal, in the
the Mediterranean Sea. The input produces a fresh- present anti-estuarine circulation pattern which in
water cap and a strong density stratification which turn, he attributed as a mechanism for sapropel de-
inhibits vertical mixing and thus the re-oxygenation position. Rohling’s proposal is supported by isotopic
of bottom waters. The resultant bottom waters would and faunal evidence from the Straits of Gibraltar and
become stagnant and anoxic (Ryan, 1972; Ryan and Sicily which suggest anti-estuarine circulation dur-
Cita, 1977; Vergnaud-Grazzini et al., 1977, 1986; ing sapropel deposition. Rohling further argued that
Williams et al., 1978; Mangini and Dominik, 1979; the present-day oligotrophic conditions are anoma-
Williams and Thunell, 1979; Muerdter et al., 1984; lous and that the Mediterranean in the past was more
Thunell and Williams, 1989). productive. δ15 N evidence generated from a core
Recent work carried out using the high-resolution recovered from the Nile Cone indicates that sapro-
astronomical time-scale (Hilgen et al., 1997), conclu- pel deposition took place at a time when surface
sively indicates that sapropel formation throughout waters were enriched in nutrients whereas the or-
the late Neogene did not occur during periods of ganic-poor marls were deposited in nutrient-depleted
deglaciation and as such the glacial meltwater theory conditions similar to present-day oligotrophic con-
has now been rejected by most workers. dition (Calvert et al., 1992). Ongoing isotopic and
geochemical analyses resultant from ODP drilling
5.2.2. Large-scale fluvial inputs may substantiate these findings.
The main exponents of this theory (Rossignol-
Strick et al., 1982; Rossignol-Strick, 1983, 1985, 5.3. Productivity: a factor in sapropel formation?
1987) argue that across the mid-latitudes, precipi-
tation greatly increased in the period prior to the The alternative to water column anoxia as the
formation of S1 . This influx of freshwater issued causal mechanism for organic-rich deposition is en-
into the Mediterranean from the African highlands hanced water column productivity, i.e. there is an
via the River Nile and on arrival in the Mediter- increase in the input rate of organic matter rather
ranean resulted in the low-salinity surface layer. The than a decrease in the rate of degradation. Many
mechanism that caused the high levels of African authors have supported the theory that increased
precipitation was attributed to fluctuations in the in- primary productivity is the cause of sapropel for-
tensity of the African Monsoon season in response mation (cf. Schrader and Matherne, 1981; Thunell
to changes in the Earth’s climate as a result of and Williams, 1982; Ganssen and Troelstra, 1987;
variations in the Earth’s orbital cycle. The African Parisi, 1987; Fontugne et al., 1989; Ten Haven et al.,
Monsoon was heaviest when the northern summer 1991). Those who advocate water column productiv-
monsoon index, calculated by variation of insolation, ity, mainly based on geochemical evidence, do not
reached maximum values. This occurred during all necessarily dispute the presence of anoxia, but they
interglacials, and during two glacial periods (Rossig- argue that anoxia by itself is not the cause of organic-
nol-Strick, 1983). Other advocates of the mechanism rich deposition, merely a symptom (cf. Henrichs and
include Maldonado and Stanley (1978), Jenkins and Reeburgh, 1987; Calvert, 1990; Pedersen and Calvert,
Williams (1983) and Parisi (1987) for the Nile re- 1990; Calvert and Pedersen, 1992). The Black Sea is
gion, and Cramp and Collins (1988), Cramp et al. usually cited to support this theory since, although
(1988) and Perissoratis and Piper (1992) for the at present it is the world’s largest anoxic water body
northern borderlands (Aegean area) of the eastern (Ryan and Cita, 1977), organic-rich sediments are not
Mediterranean. accumulating (Calvert and Fontugne, 1987). Some
Rohling (1994), in an earlier review paper, fur- support for the productivity theory comes from up-
ther argued that an increase in the intensity of the welling areas where local hydrographic flow regimes
Indian Ocean summer monsoon would manifest in are neither stagnant nor anoxic (Calvert and Price,22 A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28
1983; Muller et al., 1983; Pedersen, 1983; Sarnthein of the most recently deposited organic-carbon-rich
et al., 1988; Zahn and Pedersen, 1991). There is also layer resulted in sapropelic rather than sapropels
geochemical evidence to suggest that the Holocene sensu Kidd et al. (1978).
sapropel in the Black Sea was deposited within oxic
conditions as a response to enhanced photic zone pro- 5.4. Other mechanisms?
ductivity (Calvert, 1990).
Sancetta (1994) proposed a mat-sedimentation It has also been proposed that the dissolution of
model to explain ‘pulses’ in productivity in the exposed Messinian evaporite sequences could have
Mediterranean. Her original suggestion proposed been another mechanism capable of enhancing the
that diatom mats in the Mediterranean may repre- preservation of organic matter, either by inducing
sent the deep chlorophyll maximum, inferred to have anoxia within the bottom waters or bringing about
existed during periods of sapropel formation and changes in early diagenetic processes (cf. Klinkham-
documented by foraminiferal and nannofossil stud- mer and Lambert, 1989). Proponents advocated that
ies. The mat-sedimentation model for the deposition the dissolution produces very high bottom water
of sapropels is consistent with both the presence of salinities of up to 380‰ (De Lange et al., 1990)
stratified conditions in which nutrients trapped at leading to density stratification in the bottom waters,
depth can be exploited by vertically migrating mats resulting in the development of anoxic conditions.
and is also consistent with the evidence for high This hypothesis received additional interest follow-
export production that occurred by massive sedimen- ing the discovery of similar conditions within a
tation of mats following the intermittent breakdown number of small, isolated deep water basins in the
of stratification. More recent work carried out by eastern Mediterranean in the late 1980s. At present
Pearce et al. (1998) on the siliceous assemblage the Tyro (De Lange and Ten Haven, 1983; Jongsma
preserved in S5 samples recovered during ODP leg et al., 1983; Camerlenghi and Cita, 1987), Bannock
160, builds upon the innovative techniques and find- (Aghib et al., 1991), Poseidon, Urania, Discovery
ings of Kemp and Baldauf (1993) and substantiates and Atlante basins are anoxic. It is believed that
the original hypotheses of Sancetta (1994). Silica prior to 3.5 ka BP the Kreathus basin was anoxic
preservation is rare in the Mediterranean due to the though at present the basin is oxic (Camerlenghi and
fact that most waters of the Mediterranean are silica Cita, 1987; Ten Haven et al., 1991).
under-saturated. There are major drawbacks with the dissolution
Canfield (1994) argued for a balanced approach to theory, not least of all being able to expose suffi-
the anoxia versus productivity argument and muted cient evaporite to induce anoxia on the scale required
a third scenario where the Corg in the sediments is to produce basin-wide sapropel deposition. In addi-
the product of the balance of the Corg flux and the tion, high-resolution geochemical work carried out
diluting clastic input. He argues that in the absence on the sediment deposited beneath hypersaline water
of a clastic input a high Corg flux will result in masses within the small basins (cf. Cita et al., 1991,
organic-rich sediments being deposited irrespective and papers therein) indicates that the organic-car-
of the redox state of the water. Canfield’s approach bon-rich sediments deposited in these basins have
to the problem is novel though has received lit- significantly different geochemical signatures than
tle support as a mechanism to induce basin-wide sapropels sensu stricto.
sapropel formation. S1 sapropelic evidence from the
NW Aegean would argue against Canfield’s sug- 6. Cyclicity, atmospheric circulation and
gestion where there is an increase of terrigenous organic-carbon-rich sedimentation
flux (clastic and organic detritus) during formation
(Cramp et al., 1988). Similar evidence also exists The astronomical theory of orbital forcing, ini-
from proximal basins of the eastern Mediterranean tially proposed by Croll in the nineteenth century,
(Shaw and Evans, 1984). Conversely, it could well be and elaborated upon by Milankovitch (1930), sug-
that enhanced clastic input to the proximal basins of gests that variations in the Earth’s orbit can be
the northeastern Mediterranean during the formation manifested as (1) eccentricity around the Sun on aA. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28 23
time scale of 96–100 ka C 400 ka, (2) obliquity of glacio-eustatic sea-level lowering will isolate basins
the Earth’s orbit about its axis over a 42 ka period, and lead to a totally different hydrographic regime.
and (3) precession of the equinoxes as a result of This is undoubtedly the case in other areas (Japan
‘wobble’ in the Earth’s orbit over 21 ka. This cycle Sea) where minimal falls in sea level would have
has two components, a 24 ka cycle and a 19 ka dramatic effects on the oceanographic regime of the
cycle, and these two components combine to give an basin. The importance of Milankovitch cycles in this
overall effect of a 21 ka cycle (Hayes et al., 1976). context is that it is possible to apply statistical anal-
Spectral analysis of variations in the δ18 O con- ysis to the sedimentary record of the study basins,
tent in planktonic foraminifera in the last 782 ka to establish if there is a demonstrated relationship
give four concentrations of variance that are centred between the hydrographic regime as reflected in the
at Milankovitch periodicities, periodicities which sediments, and the cycles predicted by Milankovitch
are dominated by the 100 ka periodicity in the theory.
Brunhes chron, by the 41 ka periodicity in the It is believed that the migration of the Inter Trop-
Matuyama chron, and the Gauss chron was a pe- ical Convergence Zone (ITCZ) in the upper atmo-
riod of virtually non-existent northern ice sheets sphere is closely tied to the thermal characteristics of
(Ruddiman and Raymo, 1988). This change may be both land and ocean. As such, there is an interactive
related to the rapid Plio–Pleistocene uplift of the loop between global temperature and the movement
Himalayan–Tibetan plateau, and the Transverse and and position of climate systems. It appears that the
Sierra Nevada ranges in the Western United States palaeoceanographic regime of the western basin is
(Ruddiman and Raymo, 1988). Modelling suggests sensitive to climatic ameloriations of the North At-
that the uplift of these mountains would dramatically lantic, in particular the waxing and waning of glacial
enhance meridional waves in the stationary long cycles. As such, the oceanographic regime of the
wave pattern of the Northern Hemisphere, thereby western basin could have been influenced by the
increasing the meridionality of atmospheric circula- growth and decay of ice sheets and the associated in-
tion in the Northern Hemisphere. This could have flows of Atlantic water across the Gibraltar Sill. On
resulted in larger European ice sheets by two mech- the other hand, the eastern basin appears to be very
anisms. Firstly, an increase in the ablation of snow sensitive to climate change. In particular, the nature
and ice in orbitally driven glacial episodes as a and volume of freshwater input (in particular issuing
result of deepening and moving south of the up- from the northern borderlands and the Black Sea),
per air trough present in modern mean circulation, productivity in the photic zone, and the formation of
thereby leading to greater and more prolonged in- water masses.
vasion of Arctic air into the warmer climes in the The present-day ‘Mediterranean-type’ climate is
summer months. This mechanism appears to be the due to the position of the ITCZ in the upper atmo-
most important (Birchfield et al., 1981). Secondly, sphere: its seasonal migration, and the influence that
the strengthening of the upper air ridge present the ITCZ has upon more localised pressure systems
in modern circulation over the western Atlantic, ( Fig. 6). The present-day pattern is characterised
enabling prevalence of south-southwesterly lower- by the establishment of a very stable high-pressure
atmospheric winds to keep the oceans warm during system across the Mediterranean through the sum-
ice growth, thus enabling a suitable moisture flux for mer months resulting in hot dry conditions. As the
rapid ice growth in the high latitudes (Ruddimann high-pressure system decays (movement of ITCZ)
and Kidd, 1986). the passage of southwesterly depressions influences
One of the important facets of ice sheet growth the Mediterranean basin through the winter months.
is its inextricable link to sea level (Dean and Gard- This period is associated with cooler and wetter
ner, 1986). An estimate of sea-level lowering as a weather. The position of the ITCZ is critical in de-
response to ice growth is approximately 10 m of termining this pattern. Movement of the ITCZ in the
sea lowering for each 0.01‰ depletion of δ18 O Mediterranean area does produce distinct weather
recorded in planktonic foraminiferal assemblages patterns at present. It seems likely that a more accen-
(Fairbanks and Mathews, 1978). In extreme cases, tuated migration of the ITCZ could have large-scale24 A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28 Fig. 6. The distribution of surface pressure, winds and precipitation levels for the Mediterranean and North Africa during January and July. The average position of the Subtropical and Easterly Jet Streams (>), together with the Inter Tropical Convergence Zone (ITCZ) and Mediterranean Front (MF), H denotes areas of high pressure, L, areas of low pressure. Figure compiled and adapted from Weather in the Mediterranean Meteorological Office (1962) and Barry and Chorley (1976).
A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28 25
Fig. 7. Schematic diagram depicting climate and water mass circulation for the present-day Mediterranean basin during the summer.
WMDM D Western Mediterranean Deep Water; EMDW D Eastern Mediterranean Deep Water; MIW D Mediterranean Intermediate
Water; LIW D Levantine Intermediate Water. Values and isolines are salinity values (ppt). Figure is adapted from Wust (1961) and Emeis
et al. (1996).
effects on the regional climate and oceanographic across the sill of Gibraltar and a inflow of less saline
circulation. cooler Atlantic water above the outflow. In general,
At present, an anti-estuarine water circulation ex- fluvial inputs to the entire basin are minimal. Minor
ists within the Mediterranean (Fig. 7). This circula- shifts in the position of the ITCZ could provide the
tion pattern is driven by a number of interactive fac- mechanism to tip what appears to be a very sen-
tors though the most important are the climate and sitive climatic=oceanographic system. As such, the
bathymetry of the region. Across the eastern basin scenario proposed by Rohling (1994) and others may
evaporation exceeds precipitation. This inequality provide, in part, the mechanism to enhance water
drives the flows across much of the basin and pro- column productivity in the eastern basin and enhance
duces an outflow of saline warm water at depth the resultant flux of carbon to the seafloor.26 A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28
7. Future research Bradley, W.H., 1938. Mediterranean sediments and Pleistocene
sea levels. Science 88, 376–379.
We need to evaluate the possible links between Calvert, S.E., 1990. Geochemistry and origin of the Holocene
sapropel in the Black Sea. In: Ittekkot, V., Kempe, S.,
individual depositional basins. For example, can the Michaelis, W., Spitzy, A. (Eds.), Facets of Modern Biogeo-
sapropels recovered from the Mediterranean be cor- chemistry. Springer, Berlin, pp. 326–352.
related with similar events in the Japan Sea? Is there Calvert, S.E., Fontugne, M.R., 1987. Stable carbon isotopic ev-
an over-riding astronomical ‘fingerprint’ which is idence for the marine origin of the organic matter in the
common to a number of basins? Many questions Holocene Black Sea Sapropel. Chem. Geol. 66, 315–322.
Calvert, S.E., Pedersen, T.F., 1992. Organic carbon accumula-
remain unresolved and may remain that way for tion and preservation in marine sediments: how important is
some time. Despite six legs of international drilling anoxia? Productivity, Accumulation and Preservation of Or-
in the Mediterranean Sea, most of our long contin- ganic Matter in Recent and Ancient Sediments. Columbia
uous samples are taken from an east–west-trending University Press, New York, pp. 231–263.
‘corridor’ along the central Mediterranean —we still Calvert, S.E., Price, N.B., 1983. Geochemistry of Namibian Shelf
sediments. In: Suess, E., Thiede, J. (Eds.), Coastal Upwelling;
do not have a deep temporal north–south transect of
its Sedimentary Record, Part A. Plenum, New York, pp. 337–
core material running for example, from the Aegean 375.
to the Nile Cone— perhaps a fundamental area in Calvert, S.E., Nielsen, B., Fontugne, M.R., 1992. Evidence from
helping to resolve the formation problem once and nitrogen isotope ratios for enhanced productivity during for-
for all! Progress is undoubtedly being made and no mation of eastern Mediterranean sapropels. Nature 359, 223–
225.
doubt we will increase in our understanding of de-
Camerlenghi, A., Cita, M.B., 1987. Setting and tectonic evolution
positional processes and correlation as a result of the of some eastern Mediterranean deep-sea basins. Mar. Geol. 75,
ongoing work resultant from the 1995 ODP activity 31–55.
in the area (cf. Emeis et al., 1998; Comas et al., Canfield, D.E., 1994. Factors influencing organic carbon preser-
1998). vation in marine sediments. Chem. Geol. 114, 315–329.
Cita, M.B., Vergnaud-Grazzini, C., Robert, C., Chamley, H.,
Ciaranfi, N., d’Onofri, S., 1977. Paleoclimatic record of a long
Acknowledgements deep sea core from the eastern Mediterranean. Quat. Res. 8,
205–235.
Adrian Cramp and Gerard O’Sullivan thank fel- Cita, M.B., Broglia, C., Maliverno, A., Spezzibottiani, G.,
low shipboard scientists and technicians on board Tomadin, L., Violanti, D., 1982. Late Quaternary pelagic
ODP legs 160 and 161, Captains A. Ribbens and sedimentation on the southern Calabrian Ridge and western
Mediterranean Ridge, eastern Mediterranean. Mar. Micropale-
E.G. Oonk, drillers and crew for a professional ontol. 7, 135–162.
job well-done, and the Natural Environment Re- Cita, M.B., De Lange, G.J., Olausson, E., 1991. Anoxic basins
search Council (UK) for continuing financial sup- and sapropel deposition in the eastern Mediterranean: past and
port for UK participation in the ODP. The origi- present. Mar. Geol. 100, 1–4.
nal manuscript benefited from helpful, thorough and Comas, M., Zahn, R., Klaus, A. et al., 1996. Proc. ODP Init.
Results 161, 1023 pp.
constructive comments provided by Elisabetta Erba Comas, M., Zahn, R., Klaus, A. et al., 1998. Proc. ODP Sci.
and an anonymous reviewer. Jenny Pike provided ob- Results 161, in press.
jective comment on the use of commas and the final Cramp, A., Collins, M.B., 1988. A late Pleistocene–Holocene
review draft, and Tony Ramsay aided with printing. sapropel layer in the NW Aegean Sea, eastern Mediterranean.
Geo-Mar. Lett. 8, 19–23.
Cramp, A., Collins, M.B., West, R., 1988. Late Pleistocene–
References Holocene sedimentation in the NW Aegean Sea: a paleocli-
matic paleoceanographic reconstruction. Palaeogeogr., Palaeo-
Aghib, F.S., Bernoulli, D., Weissert, H., 1991. Hardground for- climatol., Palaeoecol. 68, 61–77.
mation in the Bannock Basin, eastern Mediterranean. Mar. De Lange, G.J., Ten Haven, H.L., 1983. Recent sapropel forma-
Geol. 100, 103–113. tion in the eastern Mediterranean. Nature 305, 797–798.
Barry, R.G., Chorley, R.J., 1976. Atmosphere Weather and Cli- De Lange, G.J., Middelburg, J.J., Van der Weijden, C.H., Cata-
mate. Methuen, London, 432 pp. lano, G., Luther III, G.W., Hydes, D.J., Woittiez, J.R.W.,
Birchfield, G.E., Wertman, J., Lunde, A., 1981. A palaeoclimate Klinkhammer, G.P., 1990. Composition of anoxic hypersaline
model of Northern Hemisphere ice sheets. Quat. Res. 15, 126– brines in the Tyro and Bannock Basins, eastern Mediterranean.
142. Mar. Chem. 31, 63–88.A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28 27
Dean, W.E., Gardner, J.V., 1986. Milankovitch cycles in Neogene Grazzini, C., Zachariasse, W.J., 1993. Late Quaternary central
deep sea sediment. Paleoceanography 1, 539–553. Mediterranean Biochronology. Mar. Micropaleontol. 21, 169–
Demaison, G.J., Moore, G.T., 1980. Anoxic environments and 189.
oil source bed genesis. Am. Assoc. Pet. Geol. Bull. 64 (8), Kemp, A.E.S., Baldauf, J.G., 1993. Vast Neogene laminated
1179–1209. diatom mat deposits from the eastern equatorial Pacific Ocean.
Emeis, K.C., Mycke, B., Degens, E.T., 1990. Provenance and Nature 362, 141–143.
maturity of organic carbon in Late Tertiary to Quaternary Kidd, R.B., Cita, M.B., Ryan, W.B.F., 1978. Stratigraphy of
sediments from the Tyrrhenian Sea (ODP Leg 107=Holes eastern Mediterranean sapropel sequences recovered during
652A and 654A). Proc. ODP, Sci. Results 107, 545–578. Leg 42A and their paleoenvironmental significance. Init. Rep.
Emeis, K.C., Camerlenghi, A., McKenzie, J.A., Rio, D., DSDP 42A, 421–443.
Sprovieri, R., 1991. The occurrence and significance of Pleis- Killops, S.D., Killops, V.J., 1993. An Introduction to Organic
tocene and Upper Pliocene sapropels in the Tyrrhenian Sea. Geochemistry. Longman, Harlow, 265 pp.
Mar. Geol. 100, 155–182. Klinkhammer, G.P., Lambert, C.E., 1989. Preservation of organic
Emeis, K.C., Robertson, A.H.F., Richter, C. et al. (Eds.), 1996. matter during salinity excursions. Nature 339, 271–274.
Proc. ODP, Init. Results 160, 972 pp. Kullenberg, B., 1952. On the salinity of the water contained
Emeis, K.C., Robertson, A.H.F., Richter, C., Camerlenghi, A. et in marine sediments. Goteborgs K. Vetenskaps. Vitt-Sambal.
al. (Eds.), 1998. Proc. ODP, Sci. Results. 160, in press. Handl. 6, 3–37.
Fairbanks, R.G., Mathews, R.K., 1978. The marine oxygen iso- Lourens, L.J.A., 1998. Astronomical calibration of sapropels in
tope recovered in Pleistocene coral, Barbados, West Indies. ODP cores 967 and 969: implications for the chronostrati-
Quat. Res. 10, 181–196. graphic frame of the Vrica section. Proc. ODP, Sci. Results
Fontugne, M.R., Paterne, M., Calvert, S.E., Murat, A., Guis- 160, in press.
chard, F., Arnold, M., 1989. Adriatic deep water formation Lourens, L.J.A., Antonarakou, F.J., Hilgen, F.J., Van Hoof,
during the Holocene; implications for the reoxygenation of the
A.A.M., Vergnaud-Grazzini, C., Zachariasse, W.J., 1996. Eval-
deep Mediterranean Sea. Paleoceanography 4, 199–206.
uation of the Plio-Pleistocene astronomical timescale. Palaeo-
Ganssen, G., Troelstra, S.R., 1987. Paleoenvironmental changes
ceanography 11, 391–431.
from stable isotopes in planktonic foraminifera from eastern
Luz, B., 1979. Paleoceanography of the post-glacial eastern
Mediterranean sapropels. Mar. Geol. 75, 221–230.
Mediterranean. Nature 278, 847–848.
Hayes, D., Imbrie, J., Shackleton, N.J., 1976. Variations in the
Maldonado, A., Stanley, D.J., 1978. Nile Cone depositional pro-
Earth’s orbit: pacemaker for the ice ages. Science 194, 1121–
cesses and patterns in the Late Quaternary. In: Stanley, D.J.,
1132.
Kelling, G. (Eds.), Sedimentation in Submarine Canyons, Fans
Henrichs, S.M., Reeburgh, W.S., 1987. Anaerobic mineralization
and Trenches. Dowden, Hutchinson and Ross, Stroudsburg,
of marine sediment organic matter: rates and the role of anaer-
PA, 365 pp.
obic processes in the oceanic carbon economy. Geomicrobiol.
J. 5 (34), 191–237. Mangini, A., Dominik, J., 1979. Late Quaternary sapropel on
Higgs, N.C., Thomson, J., Wilson, T.R.S., Croudace, I.W., 1994. the Mediterranean Ridge: U budget and evidence for low
Modification and complete removal of eastern Mediterranean sedimentation rates. Sediment. Geol. 23, 113–125.
sapropels by postdepositional oxidation. Geology 22, 423– Mangini, A., Dominik, J., 1982. The problem of missing sapro-
426. pels in SW Ionian Sea (Core TR 171-20). Mar. Geol. 45,
Hilgen, F.J., 1991a. Astronomical calibration of Gauss to M1–M8.
Matuyama sapropels in the Mediterranean and implication Meteorological Office, 1962. Weather in the Mediterranean, I.
for the geomagetic polarity time scale. Earth Planet. Sci. Lett. General Meteorology (2nd ed.). H.M.S.O., London, 362 pp.
104, 226–244. McCoy, F.W., 1974. Late Quaternary Sedimentation in the East-
Hilgen, F.J., 1991b. Extension of the astronomically calibrated ern Mediterranean Sea. Unpubl. Ph.D. Thesis, Harvard Univ.,
(polarity) time scale to the Miocene=Pliocene boundary. Earth MA, 132 pp.
Planet. Sci. Lett. 107, 349–368. Milankovitch, M., 1930. Mathematische Klimalehre und as-
Hilgen, F.J., Krijgsman, W., Langereis, C.G., Lourens, L.J., 1997. tronomische Theorie der Klimaschwankungen. In: Koppen,
Breakthrough made in dating the geological record. Am. Geo- W., Geiger, R. (Eds.), Handbuch der Klimatologie 1 (A).
phys. Union 78, EOS Trans. 28, pp. 287–289. Borntraeger, Berlin, 176 pp.
Jenkins, J.A., Williams, D.F., 1983. Nile water as a cause of Muerdter, D.R., Kennett, J.P., Thunell, R.C., 1984. Late Qua-
eastern Mediterranean sapropel formation; evidence for and ternary sapropel sediments in the eastern Mediterranean Sea:
against. Mar. Micropaleontol. 9, 521–534. faunal variations and chronology. Quat. Res. 21, 385–403.
Jongsma, D., Fortuin, A.R., Huson, W., Troelstra, S.R., Klauer, Muller, P.J., Erlenkeuser, H., von Grafenstein, R., 1983. Glacial–
G.T., Peters, J.M., 1983. Discovery of an anoxic basin within interglacial cycles in oceanic productivity inferred from or-
the Strabo Trench, eastern Mediterranean. Nature 305, 795– ganic carbon contents in eastern north Atlantic sediment cores.
796. In: Thiede, J., Suess, E. (Eds.), Coastal Upwelling, Part B.
Jorissen, F.J., Asioli, A., Borsetti, A.M., Capotondi, L., Devisser, Plenum, New York, pp. 365–398.
J.P., Hilgen, F.J., Rohling, E.J., Vanderborg, K., Vergnaud- Nolet, G.J., Corliss, B.H., 1990. Benthic foraminiferal evidence28 A. Cramp, G. O’Sullivan / Marine Geology 153 (1999) 11–28 for reduced deep-water circulation during sapropel deposition latitudes: influence on CO2 reservoirs of the deep ocean and in the eastern Mediterranean. Mar. Geol. 94, 109–130. atmosphere during the last 21,000 years. Palaeoceanography Oggioni, E., Zandini, L., 1987. Response of benthic foraminifera 3, 361–399. to stagnant episodes — a quantitative study of Core BAN Schrader, H., Matherne, A., 1981. Sapropel formation in the 81-23, eastern Mediterranean. Mar. Geol. 75, 241–261. eastern Mediterranean Sea: evidence from preserved opal as- Olausson, E., 1961. Studies in deep-sea cores. Rep. Swed. Deep- semblages. Micropaleontology 27, 191–203. Sea Exped. 1947–1948 8, 337–391. Shackleton, N.J., Crowhurst, S., Hagleberg, T., Pisias, N.G., Parisi, E., 1987. Carbon and oxygen isotope composition of Schneider, D.A., 1995. A new late Neogene time scale: ap- Globerinoides ruber in two deep sea cores from the Levantine plications to Leg 138 sites. Proc. ODP, Sci. Results 138, 73– Basin (eastern Mediterranean). Mar. Geol. 75 (1234), 201– 101. 219. Shaw, H.F., Evans, G., 1984. The nature distribution and ori- Pearce, R.B., Kemp, A.E.S., Koizumi, I., Pike, J., Cramp, A., gin of a sapropelic layer in sediments of the Cilicia Basin, Rowlands, S.J.A., 1998. Lamina-scale, SEM-based study of a north-eastern Mediterranean. Mar. Geol. 61, 1–12. late Quaternary diatom-ooze sapropel from the Mediterranean Stanley, D.J., 1978. Ionian Sea sapropel distribution and late Ridge, ODP site 971. Proc. ODP, Sci. Results 160, in press. Quaternary palaeoceanography in the eastern Mediterranean. Pedersen, T.F., 1983. Increased productivity in the eastern equa- Nature 274, 149–151. torial Pacific during the last glacial maximum (19,000 to Stanley, D.J., Maldonado, A., 1977. Nile Cone: Late Quaternary 14,000 yr B.P.) Geology 11, 16–19. stratigraphy and sediment dispersal. Nature 266, 129. Pedersen, T.F., Calvert, S.E., 1990. Anoxia vs. productivity: what Strohle, K., Kron, M.D., 1997. Evidence for the evolution of controls the formation of organic-carbon-rich sediments and an oxygen minima layer at the beginning of S1 sapropel sedimentary rocks? Am. Assoc. Pet. Geol. Bull. 74, 454–466. deposition in the eastern Mediterranean. Mar. Geol. 140, 231– Perissoratis, C., Piper, D.J.W., 1992. Age, regional variation and 236. shallowest occurrence of S1 sapropel in the Northern Aegean Ten Haven, H.L., De Lange, G.J., McDuff, R.E., 1991. Inter- Sea. Geo-Mar. Lett. 12, 49–53. stitial water studies of Late Quaternary eastern Mediterranean Rohling, E.J., 1994. Review and new aspects concerning the sediments with emphasis on early diagenetic reactions and formation of eastern Mediterranean sapropels. Mar. Geol. 122, evaporitic salt influences. Mar. Geol. 75, 119–136. 1–28. Thunell, R.C., Williams, D.F., 1982. Paleoceanographic events Rossignol-Strick, M., 1983. African monsoons, an immediate associated with termination 11 in the eastern Mediterranean. response to orbital insolation. Nature 304, 46–49. Oceanol. Acta 5, 229–233. Rossignol-Strick, M., 1985. Mediterranean Quaternary sapropels: Thunell, R.C., Williams, D.F., 1989. Glacial–Holocene salinity an immediate response of the African monsoon to variation of changes in the Mediterranean Sea: hydrographic and deposi- insolation. Palaeogeogr., Palaeoclimatol., Palaeoecol. 49, 237– tional effects. Nature 338, 493–496. 265. Van Harten, R., 1987. Ostracodes and the early Holocene anoxic Rossignol-Strick, M., 1987. Rainy periods and bottom water event in the eastern Mediterranean — evidence and implica- stagnation initiating brine concentrations, 1. The late Quater- tions. Mar. Geol. 75, 263–269. nary. Palaeoceanography 2, 330–360. Vergnaud-Grazzini, C., Ryan, W.B.F., Cita, M.B., 1977. Stable Rossignol-Strick, M., Nesteroff, W., Olive, P., Vergnaud-Grazz- isotope fractionation, climate change and episodic stagnation ini, C., 1982. After the deluge: Mediterranean stagnation and in the eastern Mediterranean during the late Quaternary. Mar. sapropel formation. Nature 295, 105–110. Micropaleontol. 2, 353–370. Ruddimann, W.F., Kidd, R.B., the Scientific Party of Leg Vergnaud-Grazzini, C., Devaux, M., Znaidi, J., 1986. Stable iso- 94, 1986. Introduction, background and explanatory notes, tope ‘anomalies’ in Mediterranean Pleistocene records. Mar. D.S.D.P. Leg 94, North Atlantic Ocean. Init. Rep. DSDP 94, Micropaleontol. 10, 35–69. 5–19. Wasmund, E., 1930. Bitumen, sapropel und gyttya. Geol. Foren. Ruddiman, W.F., Raymo, M.E., 1988. Northern Hemisphere Stockholm Forh. 52, 315–350. glacial regimes during the past 3 Ma: possible tectonic con- Williams, D.F., Thunell, R.C., 1979. Faunal and oxygen isotopic nections. Philos. Trans. R. Soc. London 318, 411–430. evidence for surface water salinity changes during sapropel Ryan, W.B.F., 1972. Stratigraphy of Late Quaternary sediments formation in the eastern Mediterranean. Sediment. Geol. 23, in the Eastern Mediterranean. In: Stanley, D.J. (Ed.), The 81–93. Mediterranean Sea. Dowden, Hutchinson and Ross, Strouds- Williams, D.F., Thunell, R.C., Kennett, J.P., 1978. Periodic fresh- burg, PA, pp. 1–765. water flooding and stagnation of the Eastern Mediterranean Ryan, W.B.F., Cita, M.B., 1977. Ignorance concerning episodes Sea during the Late Quaternary. Science 201, 252–254. of ocean-wide stagnation. Mar. Geol. 23, 193–215. Wust, G., 1961. On the vertical circulation of the Mediterranean Sancetta, C., 1994. Mediterranean sapropels: seasonal stratifica- Sea. J. Geophys. Res. 66, 3261–3271. tion yields high production and carbon flux. Palaeoceanogra- Zahn, R., Pedersen, T.F., 1991. Late Pleistocene evolution of phy 9, 195–196. surface and mid-depth hydrography at the Oman Margin: Sarnthein, M., Winn, K., Duplessy, J.-C., Fontugne, M., 1988. planktonic and benthic isotope records at ODP Site 724. Proc. Global variations of surface ocean productivity in low and mid ODP, Sci. Results 117, 291–308.
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