Evidence for the presence of an oxygen depleted Sapropel

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Evidence for the presence of an oxygen depleted Sapropel Intermediate Water across the Eastern Mediterranean during Sapropel S1 Eleen Zirks, Michael D Krom, Dongdong Zhu, Gerhard Schmiedl, and Beverly N Goodman-Tchernov ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.9b00128 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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ACS Earth and Space Chemistry

Evidence for the presence of an oxygen depleted Sapropel Intermediate Water across the Eastern Mediterranean during Sapropel S1 Eleen Zirks1, Michael D. Krom1,2*, Dongdong Zhu1, Gerhard Schmiedl3, Beverly N. GoodmanTchernov1 1 Leon H. Charney School of Marine Sciences, University of Haifa, Israel 2 Morris Kahn Marine Research Centre, Department of Marine Biology, Charney School of Marine Sciences, University of Haifa, Israel; and School of Earth and Environment, Leeds University, Leeds, UK 3 Center for Earth System Research and Sustainability, Institute for Geology, University of Hamburg, Germany KEYWORDS Redox sensitive trace metal, benthic foraminifera, circulation, water masses, Holocene ABSTRACT

In the Eastern Mediterranean Sea (EMS), natural climate-driven changes that impacted its physical circulation and created deep-water anoxia have led to periods of distinctive deposits referred to as

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sapropels. The most recent sapropel event (S1) occurred between ~6-10 kyr BP. Use of a global climate model has suggested that there was a previously unrecognized mid-depth (500-1800m) water mass present at that time. Here, a broad review of cores previously analyzed from across the EMS was undertaken to determine if the field evidence was compatible with the presence of this intermediate water mass. The proxy records document a widespread oxygenated interruption centered around 8.2 ka BP in water depths between 500 and 1800m, but not present in shallower or deeper water. We attribute this pattern to the formation and spread of a water mass, which we refer to as Sapropel Intermediate Water (SIW). It is shown that this water mass was formed in the Aegean Sea and became more depleted in oxygen, to anoxicity but not euxinicity, as it flowed west to the Adriatic and east across the Levantine basin. The rate of formation and flow of this water mass was estimated to be slow but not zero. An evolving Oxygen Minimum Zone in the SE Levantine is posited where oxygen was depleted first in shallower water and expanded with time into deeper water. The presence of SIW obviates the need for the previously suggested thin anoxic blanket, since the 1300m thick SIW can be fully deoxygenated in 200-500 years.

INTRODUCTION The Eastern Mediterranean Sea (EMS) has regularly experienced oceanographic changes in the recent geological past.1,2 During past warm and humid intervals, sapropel sediments containing high levels of organic matter were deposited, which contrasts with the organic-depleted sediments deposited under the present oligotrophic and well-ventilated conditions.3,4 Sapropels are dark sediments with high organic content and are found in many deep-sea cores of the EMS.2–5 The most recent period of sapropel deposition is called sapropel S1, and is defined by increases in Ba/Al and TOC ratio and was deposited between ~10.8 and 6.1 kyr BP.2 A physical model for circulation during S1, proposed a 3-layer water column similar to the present day with surface


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water flowing into the EMS through the Straits of Sicily and Levantine Intermediate Water (LIW) flowing westward out of the basin.6 It was suggested that below the upper 500m there was a single water mass which became stagnant and anoxic.6 In contrast to these previous models, it was suggested recently, that there was an mid-depth water interval between 500m and 1800m that was partially or intermittently ventilated and which was reduced in oxygen but was not as fully stagnant.7 In the following, we refer to this proposed water mass as Sapropel Intermediate Water (SIW). In the modelling study from 2015, a regional setup of the Max Planck Institute ocean model (MPIOM) coupled to the Hamburg ocean carbon cycle model (HAMOCO5) was used.7 The regional model setup covered the entire Mediterranean Sea, the Black Sea and a small part of the North Atlantic Ocean with a horizontal resolution of around 20km and 29 vertical levels.8,9 The global climate model were performed with the atmosphere-ocean-dynamical vegetation model ECHAM5/MPI-OM/LPJ.7,10 Various sensitivity experiments for the early Holocene (9 ka BP) were used to test the current hypotheses for sapropel formation. The experiments demonstrated that neither enhanced surface-water productivity nor stagnating deep-waters induced by enhanced Nile river runoff in combination with high nutrient loads can account for sapropel 1 formation alone. While deep-water anoxia developed only after a long period of stagnation since at least the late glacial Heinrich event 1, the water column above 1800m still received sufficient oxygen through repeated ventilation events under early Holocene boundary conditions.7 A difference in sediments deposited above 1800m has been recognized previously in both biological (e.g. benthic foraminifera abundances) and geochemical properties (e.g. lower Ba and organic C flux preserved in the sediment) but this was not recognized as a uniquely different water mass.2,4 At present, the EMS oxygen content changes with distance from deep water mass sources are relatively small.11 However, these circumstances given the much lower oxygen status of the water column during S1


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deposition might result in the water column passing through important redox states such as hypoxia and anoxia, that can involve ferruginous or euxinic conditions. An additional paleoenvironmental effect on the water column properties in the SE Levantine basin, is the known increased Nile flood during S1.1 During the S1 period, the African monsoon was broadened, resulting in the African Humid Period which occurred from ~11.7 to 5 kyr BP.12,13 The increased rainfall caused a major increase in the magnitude of the annual Nile flood.1,14–16 Resulting major phytoplankton blooms would be expected to interact with, and further reduce, the oxygen content of the SIW, with the effect greatest close to the Nile delta and decreasing to the north and west in the direction of Nile flood.17 The number and species assemblages of benthic foraminifera are often used to estimate the oxygen status of the overlying water during deposition. A commonly used parameter is the Shannon-Weaver diversity index H(S).18 In the Mediterranean Sea, a new Oxygen Index (OI) was introduced and applied to several S1 sediment cores.19,20 It is calculated by the proportions of high oxygen and low oxygen indicator species and the diversity as a value between 0 to 1. High values (>0.5) indicate well oxygenated conditions and low values (500m) and have a residence time of 100–150y.11 While the EMDW forms in the S. Adriatic and flows to the east, the CDW formed in the Aegean and flowed both to the east into the Levantine basin and west into the Ionian basin and Adriatic Sea. The EMDW eventually mixes up into the LIW and flows out of the basin into the Western Mediterranean Sea. As the EMDW flows away from its source in the S. Adriatic it gradually loses dissolved oxygen and accumulates dissolved nutrients (nitrate, phosphate and silicate).11,31 CDW behaves in a similar manner but the effect is much smaller because it is the densest water mass at present and only a small amount of labile organic matter reaches those depths. The primary productivity levels in the present EMS are among the lowest values found anywhere in the global ocean with an average of 60-80 gC m-2 y-1.32 A complimentary estimate of the average primary productivity based on SEAWIFS satellite data (121 gC m-2 y-1) was calculated using a 4year average of SEAWIFS data.33 It has been suggested that despite being a semi-enclosed inland sea, the EMS has many of the characteristics of an ocean gyre.26 As a result of these ultraoligotrophic conditions in EMS surface waters at present, there is comparatively high dissolved oxygen content throughout the intermediate and deep water column (~180 mmolesO2/kg or higher) and very low organic matter preserved in the sediment (~0.2% TOC).2,34 Model experiments suggested the persistence of an anti-estuarine circulation in the EMS also during the deposition of S1 with similar surface and intermediate water masses (i.e. above ~500m).6 This anti-estuarine thermohaline flow was somewhat slower than the present flow but still resulted in a net export of dissolved nutrients.35 These models assumed a single deep-water


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mass that became uniformly anoxic/euxinic during most of S1.35,36 The possible effect of the 8.2 ka BP global cooling event has been modelled and it was suggested that there was a short term ventilation of the water column to a depth of 1300m starting in the Cretan Sea.36 More recently, a regional coupled ocean-biogeochemical general circulation model (GCM) was developed, which also suggested a generally similar slower thermohaline circulation in the upper 500m as predicted earlier.7,35 However, the GCM model results also suggested a more complex and dynamic situation for the deeper layers below 500m with stagnant conditions below 1800m starting as early as ~1516 kyr BP during the Heinrich event H1. Extensive field evidence demonstrates that this water mass was persistently reduced in oxygen and became euxinic during S1 time (9.8 to 5.7 kyr BP).2,4,37 This conclusion is based on the absence of benthic foraminifera, the formation of pyrite with relatively low δ34S within the upper few centimeters indicating the presence of anoxic sulphidic conditions at or close to the sediment-water interface, Fe and Mo isotope and redox sensitive trace metal systematics suggest a euxinic water column, the rapid degradation of organic matter when the sediment was exposed to oxic conditions after the end of sapropel, and the preservation of delicate dinoflagellate cysts and tree pollen.2,38,39 However, in contrast to the earlier model, in 2015 the GCM model suggested that the depth interval between approximately 500 and 1800m was intermittently ventilated and was a different water mass from that above 500m and below 1800m.7,35 Analogous to modern EMDW, it is probable that this intermediate water mass, became mixed into the LIW (200–500m) and advected out of the basin. While the model from 2015 did not consider in detail the changes in the intermediate water mass, it was predicted that during the Younger Dryas and during the 8.2 ka global cooling event there was enhanced partial ventilation at these mid-depths, while the deep water mass remained stagnant.7


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RESULTS Observations from cores sampled across the EMS In this study, the published results from a series of sediment cores collected between 500 and 1800m across the entire EMS were compiled and compared with data from cores collected both above 500m and below 1800m. Given that not all studies performed the same set of analyses, we concentrated on cores which contained proxy measurements that could be related to the oxygen status of the overlying water. The most widespread, and at this time useful proxy measurements for the oxygen status of the overlying water are the abundance, species composition and diversity of benthic foraminifera and concentrations of redox sensitive trace metals. Radiocarbon ages and age profiles were carefully assessed. In those few cores where un-calibrated radiocarbon ages were given in the original reporting, the ages were recalculated as calibrated radiocarbon ages (see SI) to insure consistency across the different cores. The selected cores represent a wide geographical and depth range. Particular emphasis was made on how and when the sediment shows evidence of reduced oxygen content, hypoxia and anoxia as well as evidence of re-ventilation. This re-ventilation event, centered around 8.2 ka BP, divides the sapropel into two parts: the older S1a and the younger S1b.2 The sediment cores that we have examined in detail are shown in Figure 1 and detailed in Table SI1.


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Figure 1. Map of core locations used in this study across the EMS. The core locations in red circles are from 500-1800m depth. Core locations 1800m are shown in green. A black half circle on the core location is for cores with evidence for an oxygenated interruption at ~8.2 ka BP. The cores shown with a white half circle are locations with an apparent interruption but attributed by the original authors as due to a slump. Those with a yellow half circle have incomplete data sets that cannot be used to define an interruption. Isobaths are shown at 1000, 2000, 3000 and 4000m water depths. Shallow cores (4°C in the N. Aegean area during the 8.2 ka global climate event due to the locally strong influence of the Siberian high.57 This temperature decrease was larger than the suggested decrease in the S. Adriatic.4 Such a temperature decrease could have resulted in enhanced convection and thus formation of intermediate water in the Aegean Sea during the 8.2 ka BP event. This pattern resembles the modern transient conditions of deep water formation in the EMS during the Eastern Mediterranean Transient, when Cretan Deep Water was formed in the Aegean and flowed to both the west into the Ionian basin/Adriatic Sea and in the east into the Levantine basin.29 As the Cretan Deep Water flows away from its source it becomes progressively oxygen-reduced and nutrient-enriched in a


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similar way (though at a smaller scale) to the observed changes in SIW.58 This is because Cretan Deep Water is present as a layer in the deepest EMS and hence has a low flux of labile organic matter from the surface available for microbial respiration. The observed changes in oxygenation in SIW are likely linked to changes in the rate of formation of this water mass. The oxygen content at SL123 at the exit from the Aegean Sea to the Levantine basin was 4-40 mmolesO2/m3 during S1a and approximately 80 mmolesO2/m3 during S1b based on the preferred habitat certain benthic foraminifera species dwell in.59–61 If it is assumed that the primary productivity was similar to that at present7, then it required that there was slow but not zero ventilation of SIW during S1a, increased water mass formation during the interruption (as would be expected from a global cooling event) and decreased water mass formation during S1b but not as low as during S1a. A best attempt though rough calculation for the residence time of water in the Levantine basin during S1a (~3000 years) and ~2400 years for S1b is given in the SI. At the end of S1 both the SIW and deep water were ventilated and a flow rate similar to the modern flow regime was established.

Effect of the increased Nile flood on oxygen changes in the SE Levantine Based on the analysis of the proxy records from the depth interval 500-1800m across the SE Levantine basin (Figure 2 and 3), a conceptual model for the biogeochemical processes involved has been developed (Figure 4). It is suggested that as SIW flowed from the Aegean Sea (SL123, which was hypoxic but never anoxic) across the Levantine basin, the dissolved oxygen in the SIW was progressively reduced from northwest (9501) to the southeast (PS009PC, SL112 and 9509 offshore Israel; P362/2-33, Nile delta) (Figure 3, Table SI1).15,20,47,48 Regional changes in dissolved oxygen may have been linked to export production of organic matter from the photic zone sinking into the SIW and then interacting with nutrient-rich waters entering from increased Nile flood activity.1,4 Prior to 1965 and the closure of the Aswan Dam, the annual Nile flood (July-September) jetted out into the SE Levantine basin.17 This modern Nile flood also carried increased dissolved nutrients and resulted in a pulse of increased primary productivity. This pulse expanded from the


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Nile delta mainly to the east and was seen as a jet of brown water visible as far as Beirut.62 During S1, it has been suggested that the flow of the Nile was far greater than at present.1,16 It is therefore considered likely that it would also have had increased nutrient content and primary productivity at least in the region of the easterly part of the SE Levantine basin.15 Evidence for these systematic changes are found in the geographical changes in oxygen status shown in Figure 3 where there was no S1b at core 9501, there was anoxicity but no laminations during S1a and hypoxic conditions in S1b in several cores off the Israeli coast, while the sediment in core P362/2-33 on the Nile delta was euxinic and laminated. Thus the nearer to the Nile delta, the more extreme was the oxygen depletion.15,23,48

Figure 4. Schematic diagram showing the conceptual model of water masses and circulation in the SE Levantine. The figure shows the relative magnitude of labile organic matter (green arrows) exported into Sapropel Intermediate Water with higher fluxes close to the coast due to the effect of the Nile flood (dark blue). The changes in oxygen status of the intermittently suboxic/anoxic body of water during S1 were controlled by the rate of flow of SIW and the flux of labile organic matter being respired. Sapropel Intermediate Water as an evolving oxygen minimum zone


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We have plotted in Figure 5, the beginning of S1 from eight cores from the SE Levantine basin (500-1500m). We use Ba/Al ratio, RSTM, benthic foraminifera fauna and/or TOC to define the beginning of S1. Most of the cores are dated by corrected 14C and/or wiggle matching 18O from planktonic foraminifera with Soreq cave speleothem.45,47,49,63–65 For cores MD84641, MD84639 and MD84627, it is not specifically mentioned whether the 14C measurements had been calibrated or not.49 It was assumed that the values reported were unaltered radiocarbon age measurements, since standard publication practice is to describe calibration choices or use calibration nomenclature (e.g. cal. year BP). We therefore calibrated the ages using the Marine13 curve of Calib 7.0.4, applying a standard reservoir age of 400 years (see SI for more detailed justification of this procedure).66

Figure 5. Timing of beginning (squares) of sapropel 1 and the interruption (circles) vs water depth from the SE Levantine. The proxies used for definition are shown in red: V/Al, green: Diversity


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index, black: TOC, blue: Ba/Al. The error bars represent the 2σ standard deviation of the 14C ages. For exact core locations, please see Figure 1 and table SI2. It was observed that the indicators of S1 started earlier in the shallow water (500–600m) than in the deep water (~1350m; Figure 5). The difference of the onset in the shallow to the deep is about 1100 years (10 ka BP in PS009PC and 8.9 ka BP in MD84641). In those stations where several different methods were used to identify the beginning of S1 (PS009PC, 9509, 9501), the age difference of 100-200 years within the single core, was smaller than the overall change with depth between cores. For example, the difference of the beginning of S1 in 9509 is about 200 years with an earlier onset in Ba/Al ratio compared to the subsequent onset in TOC while the start of sapropel in MD84627 200m deeper was at 8.6 ka BP. This pattern of an evolving OMZ is what would be expected for a system in which the oxygen in the water column is controlled by the water, saturated with oxygen when it was formed in the Aegean and then being modified by the in-situ decomposition of labile organic matter being exported from the photic zone. The observed rate of decrease of dissolved oxygen with depth at any given location is a combination of the amount of labile organic matter exported from above and the water mass flow of SIW across the basin. For those cores in the SE Levantine basin there was, in addition, an increased labile organic matter flux due to the increased primary productivity caused by the Nile flood plume which is likely to have caused a higher rate of oxygen consumption closer to the shoreline than further offshore. Without further information it is not possible to determine the relative magnitude of this effect. A similar pattern was observed, with the start of anoxia in S1 being observed later in deeper water.67 However, that study did not recognize the fundamental difference in behavior of the intermediate and deep water masses in S1 and considered mainly the change in oxygen status of


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the deep water mass (>1800m). This result was criticized stating that it was unable to explain the observed changes in the beginning of S5.4 However, S5 is known to be a different and fundamentally more extreme sapropel compared with S1. Thus, for example, during S5 it was shown that there were euxinic (sulphur oxidized) microbes in the photic zone which means that the surface layers above 500m were also stagnant in contrast to the oxygenated upper 500m postulated for S1.6,35,68 The suggested anoxic blanket It has been suggested that there was a relatively thin anoxic blanket covering both the deep and intermediate depths in the Eastern Mediterranean during S1.4,41 The reason that such a blanket was hypothesized was because it was calculated that the entire water column (2500m) could not switch from oxygenated to anoxic conditions quickly enough based on the observed fluctuations in the benthic foraminifera populations. Using the oxygen utilization rates for deep water of 0.5 mmoles O2 m-3 yr-1, it was calculated that it would take 700-1700 years for the water column to become fully anoxic.11,41 The observed changes in benthic foraminifera populations occurred within few hundred years. Based on the conceptual model for water circulation developed in this study, only the depth of 500–1800m (1300m) fluctuated from oxic to anoxic. It is possible here to perform a similar simple calculation. The oxygen utilization rate was modified so as to be appropriate for the depth interval from 500–1800m, and thus is ~0.7 mmoles O2 m-3 yr-1.11 Using an initial oxygen concentration of 180 mmolesO2 m-3, it would take ~200-500 years for the SIW to become anoxic. This rate of developing anoxia is compatible with measured fluctuations in oxic/anoxic conditions. The anoxic blanket hypothesis requires that labile organic matter from the photic zone is exported rapidly downwards and has minimal effect on the oxygen concentration within the water column. The organic-rich particles were hypothesized to reach the sediment water interface and there


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consume oxygen resulting in an anoxic blanket. Such a model requires that anoxia developed first in deeper water and then expands upwards – the opposite of what is proposed here (Figure 4 and 5). There is nowhere where such a pattern exists in the modern open ocean. Another problem with the anoxic blanket hypothesis is that it is incompatible with the recent data concerning redox sensitive trace metals. These redox sensitive trace metals require that there is a substantial thickness of anoxic water overlying the sediment, which is scavenged by particles accumulating additional redox sensitive trace metals, which are then deposited and build up in the sediment. A thin anoxic blanket would not accumulate the amount of such trace metals observed.15,21,22,38 CONCLUSIONS The present EMS is characterized by ultraoligotrophic conditions in the surface waters including very low primary productivity and nutrient concentrations; resulting in a water body that behaves more like an ocean gyre than an inland sea.26 Modern sediments have low organic matter concentrations (generally 0.1-0.2% organic C) and almost fully oxygenated conditions in the water column. However, in the past, it has regularly experienced alteration of physical circulation, increased nutrient discharge and expanding areas of oxygen depleted water as a result of natural climate change, during which organic-rich sapropel sediments were deposited. The most recent of these periods of sapropel deposition (S1) was between 10.8 and 6.1 kyr BP.2 Previous studies of the physical circulation of the EMS during S1 presumed a three-layer water mass structure similar to today, but with a weakened anti-estuarine circulation and stagnant deep water that became anoxic/euxinic.35,36 However, it was suggested that there was a more complex configuration.7 It was predicted an additional intermediate water mass (500-1800m) that was partially/intermittently ventilated, and which was reduced in dissolved oxygen but not fully stagnant as was the deepest waters.


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In this study proxy field evidence for the oxygen status of the past overlying water column from across the EMS was assembled. It was found that there was a characteristic pattern in sediment cores sampled at depths between 500 and 1800m. In those depths there was a strongly anoxic initial period of deposition (S1a), a characteristic interruption in sapropel deposition at 8.2 ka, coincident with the global cold period followed by a less strongly anoxic period of oxygen reduced deposition (S1b). By contrast, there was no interruption observed in deeper or shallower cores. This regional pattern was interpreted to have been caused by an intermediate water mass, the ‘Sapropel Intermediate Water’. This pattern could not have been caused by simple deep water ventilation as this was both incompatible with the modelling data and the observed lack of interruption in cores sampled above 500m. It was found that there were less extreme oxygen depletion in the Aegean Sea compared with both the SE Levantine and the Adriatic Sea (Figure 2). Based on this regional pattern it was concluded that the SIW water mass was formed in the Aegean Sea. It was suggested that the water flowed slowly away from its source during S1a and S1b and more rapidly during the 8.2 ka BP interruption. In the SE Levantine during S1, it was observed that the spread of SIW was accompanied by an evolving Oxygen Minimum Zone in which oxygen was depleted first in shallow water and expanded with time into deeper water. This pattern, which is similar to that observed in OMZ’s at present, is due to the consumption of dissolved oxygen by the breakdown of labile organic matter dropping from the photic zone above. In this case the amount of export carbon production was likely increased due to the known major increase in the magnitude of the Nile flood during S1.1 The presence of this previously unrecognized water mass removes the need for the proposed thin anoxic blanket, since the 1300m deep SIW can be fully deoxygenated in


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~200-500 years assuming similar carbon export to the present values rather than the 700-1700 years required to deoxygenate the entire deep water.41 By characterizing SIW as a water mass, and looking at its geographical and temporal variability, it is possible to suggest some important properties of this water mass. This new paradigm for deposition conditions, particularly the redox poised SIW, opens the possibility of examining the depositional patterns of a variety of redox sensitive geochemical parameters in a system in which the oxygen sensitive tracers are well preserved. Several studies have been carried out already examining the patterns of abundance and species distribution of benthic foraminifera as well as studies on redox sensitive trace metals, and their isotopes.19–23,69 Future studies should include examining the effect of changing redox distributions on redox sensitive nutrient elements such as P and Fe cycling in the water column. Such studies are important to enable predictions concerning the modern anthropogenically-driven expansion of oxygen minimum zones. These conclusions may also contribute to the understanding of the paleooceanographic conditions generally, such as those associated with geological periods of mass extinctions and Pre-Cambrian environmental settings from which life evolved. ACKNOWLEDGMENTS The authors would like to express appreciation to support received from Sir Mick Davis (BNGT) and Mr. Norman Kirscher (BNGT) for academic scholarship funding (EZ and DZ). AUTHOR INFORMATION Corresponding Author *Michael D Krom: [email protected], Morris Kahn Marine Research Centre, Department of Marine Biology, Charney School of Marine Sciences, University of Haifa, Israel


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Author Contributions The data sets were assembled and organized by EZ. The manuscript was written through contributions and discussions between all of the authors. All authors have given approval to the final version of the manuscript. SUPPORTING INFORMATION 

Includes supplementary tables SI1 and SI2

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Deeper introduction about the studied sediment cores from the EMS

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Explanation how 14C ages were corrected

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Calculation about residence time of SIW in the EMS

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Evidence for the presence of an oxygen depleted Sapropel

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