Deposition Of S1 Research Articles

Reconstructing deep-sea oxygenation in the Western Alboran Sea Basin during Late Pleistocene-Holocene (last 37 kyrs): Insights from a multiproxy approach

A deep-sea sediment record from the Western Alboran Basin spanning the last 37 kyr has been analysed using a multiproxy approach, integrating sediment colour, magnetic susceptibility, elemental ratios, benthic foraminiferal assemblages and bioturbation, gaining new insights into Alboran Basin deep-sea environments and palaeoceanographic conditions since the Last Glacial Maximum. The combination of a diverse set of proxies made it possible to recognise the diagenetic signals and reconstruct new low-frequency events in the deep basin, recording the significant fluctuations in deep water oxygenation occurred during the studied period. From the studied data, short-term periodic low-oxygen events are recognised during Heinrich Stadials 2 and 3 (HS2 and HS3) and Greenland Interstadial GI8, showing important responses of the deep sea environments to climate climate change. Furthermore, the presence of a not previously well-defined Organic Rich Layer 2 (ORL 2) can be defined from the observations as a sequence of events transitioning between low oxygen (associated with humid conditions and Heinrich Events) and oxygenated conditions, and not as a singular low oxygenation event. In addition, the Organic Rich Layer 1 (ORL1), associated with the most recent low oxygen event has been also recognised in the record, with low oxygen conditions starting as early as 16 kyr before the present (BP) during the HS1 and spanning to 8 kyr BP with important fluctuations in environmental conditions and oxygenation. Several climate events that have been previously recognised in the Northern Hemisphere and the Mediterranean Basin can be linked to the observed changes in oxygen conditions during ORL1 deposition, for instance reoxygenation that occurred during the pre-ORL1, between the demise of Heinrich Event 1.1 (HE1.1) and the end of HS1 and the inception of the Bølling-Allerød (BA); the start of ORL1 deposition during BA with the onset of low oxygen conditions. Interestingly, the BA is punctuated by short duration reoxygenation events that can be recognised in the record and correlated to cold events observed in the North Atlantic records. In addition, there is a partial discontinuous reoxygenation during the Younger Dryas (YD), observed also in the Eastern Mediterranean and in other Mediterranean sites which was followed by the return to lower-oxygen conditions after the YD, coinciding with the start of S1 deposition. Finally, the final reoxygenation event occurs at around 8 kyr BP along with the demise of the ORL1.

A high-resolution study of planktonic foraminifera during the Holocene at the Tilos-Symi sea basin in the SE Aegean Sea

High-resolution data for planktonic foraminifera and their groups of ecological interest, including Herbivores and Carnivores, combined with benthic foraminifera and a great variety of biogeochemical indices from the SE Aegean sediment core ST5, contribute to a detailed study concerning major and minor climatic episodes during Holocene. The ST5 sediment record, retrieved from the Tilos-Symi marine basin, evidences the impact of the nearby land, the local climate, and the Rhodes Gyre imprint on the eastern Mediterranean water circulation. Pronounced environmental changes are detected during a preconditioning period of ~400 years before the onset of sapropel S1 deposition and during the deposition phases S1a (10.0–8.4 ka BP) and S1b (8.0–6.1 ka BP). Major freshwater influx episodes (10.5 ka BP, 9.2–8.9 ka BP, 7.4 ka BP, 4.5–4.3 ka BP, and few additional during the Late-Holocene) are revealed, prominent oxygen deficiency time intervals (including a distinct brief anoxic period) as well as several drier, cooler and warmer climatic events. Herbivore planktonic foraminifera alternate their dominance with the Carnivores at the preconditioning period before the S1 onset and at the Late-Holocene (2.0–1.5 ka BP) when conditions of good seawater circulation, oxygenation, and productivity alternate with stratified low oxygenation waters and high freshwater influx from the land.

Marine nitrogen cycling dynamics under altering redox conditions: Insights from deposition of sapropels S1 and the ambiguous S2 in the Eastern Mediterranean Sea

The eastern Mediterranean Sea (EMS) sedimentary record is periodically interspersed with organic-rich ‘sapropel’ layers. Sapropels are characteristic of basin-wide anoxic events, triggered by precession-forced insolation maxima. Relatively subdued insolation maxima, however, are not always expressed as distinct sapropel events. The EMS sedimentary record is thus useful to investigate feedbacks between marine anoxia and the nitrogen (N) cycle and offers an analogue for modern deoxygenation and past oceanic anoxic events. To this end, we investigated a ∼68 kyr sedimentary record from the EMS containing the well-established sapropel S1 (deposited in two phases: S1a [∼10.5–8.5 ka BP] and S1b [∼7.8–6.1 ka BP]) and sediments timed to the ambiguous S2 sapropel (∼53 ka BP). We used lipid biomarkers of microorganisms to reconstruct key N-cycle components: (1) anaerobic ammonium oxidation (anammox) using ladderanes and a stereoisomer of bacteriohopanetetrol (BHT-x), (2) dinitrogen gas (N2) fixation using heterocyte glycolipids, and (3) nitrification by Thaumarchaeota using crenarchaeol. Additionally, benthic foraminifera and trace metals (U, Mo, Mn) were used to reconstruct redox conditions. During S1a, abundances of Thaumarchaeota increased, likely promoted by elevated high-nutrient freshwater discharge. At this time, a combination of phosphorus supply and intensified loss of bioavailable N via water column anammox, may have reinforced anoxia by favoring diatom-diazotroph associations. During S1b, anammox is equally intense. Yet, no positive feedback on N2-fixation is observed, likely because diazotrophs were phosphorus limited. Instead, anammox may have provided negative feedback on anoxia by quenching primary production. Ladderanes suggest additional episodes of anammox between ∼69 to 39 cal ka BP, corresponding to brief periods of water column deoxygenation. Anoxia likely occurred at the sediment–water interface in S2-timed sediments (53–51 cal ka BP). During these episodes, ladderanes co-occur with the later eluting BHT-34R stereoisomer. δ13CBHT-34R indicate an anammox source, potentially synthesized by marine sedimentary anammox bacteria. No corresponding increase in diatom-diazotroph associations is observed, likely due to the oligotrophic conditions and the limited effect of sedimentary anammox on N-availability in the euphotic zone. Our results highlight various modes of operation of the N-cycle at different degrees of deoxygenation, which depend amongst others on nutrient-availability and the niche-segregation of N-loss and N2-fixating microorganisms.

High-frequency modification of the central Mediterranean seafloor environment over the last 74 ka

Here we present a high-resolution record of benthic foraminiferal assemblages for the last 74 kyr from the Sicily Channel Ocean Drilling Program Leg 160 Site 963. Benthic foraminiferal results are compared with geochemical (benthic and planktic δ 18 O and δ 13 C) and calcareous plankton data, previously acquired on the same marine core sediments. Within the succession, three benthic foraminifera compositional zones were defined. Temporal changes in the assemblages are interpreted in the context of the modification of subtropical and temperate climate systems that affected the Mediterranean thermohaline circulation. A close connection between bottom conditions in the Sicily Channel and eastern Mediterranean Sea is evident in two intervals, characterized in the ODP Site 963 by reduced oxygen conditions. The first one, around 53–51 ka, is tentatively attributed to the ‘missing’ sapropel S2 while the second, between 35 and 29 ka, is marked by short and recurrent episodes of bottom-water oxygen decrease. Both are related to a weakening of the intermediate circulation in the Sicily Channel connected with relatively high northern hemisphere summer insolation and increase in Nile River discharge, which inhibited vertical mixing and intermediate water ventilation in Eastern Mediterranean Sea. Over the last deglaciation, the African humid period (AHP) and the sea level rise, also influenced the water mass structure in the Sicily Channel and a reduction of the bottom ventilation is suggested by a strong reduction of deep-water Miliolids. Decreased bottom oxygen levels, which testifies for a weakening of intermediate circulation in Sicily Strait, also characterized the interval corresponding to S1 deposition. Yet, the strong decrease of benthic foraminiferal abundance related to a low surface water trophic level, appears to be conditioned by the reduction of trophic levels in the western Mediterranean. The very high relative abundance of U. mediterranea recorded during this interval is explained by the availability of organic matter during a limited short year period and/or the availability of more degraded organic matter from river runoff. In general, compositional data analysis highlighted a quite complex response of benthic foraminifera to paleoclimatic changes. However, changes in the benthic foraminiferal assemblages recorded during the last glacial are coherent with surface paleoproductivity dynamics connected with D-O oscillation, and support oligotrophic, meso-eutrophic and oligo-mesotrophic conditions during the early interstadials, late interstadials and stadials, respectively. • High-resolution ODP963 (Sicily Channel) Last Glacial to Holocene benthic record. • Dansgaard-Oeschger imprints in bottom environments. • Connection between western and eastern Mediterranean dynamics. • Relationships between Levantine Intermediate Water flow and bottom ventilation.

Surface hydrographic changes at the western flank of the Sicily Channel associated with the last sapropel

In the eastern Mediterranean Sea, the early Holocene was characterized by major climatic and oceanographic changes that led to the formation of the last sapropel (S1) between 10.8 and 6.1 kyr cal. BP. These hydrographic changes might have altered the water exchange between the eastern and western Mediterranean sub-basins through the Strait of Sicily, but the existing evidences are inconclusive. In the present study we show new evidence from sediment core NDT-6-2016 located at the western flank of the Sicily channel, a key location to monitor the surface/intermediate water exchange between the two Mediterranean sub-basins. We perform paleo-hydrographic reconstructions based on planktic foraminifera ecology for the last 15 kyr cal. BP, including the S1 deposition interval. In addition, δ 18 O measurements in both Globigerina bulloides and Globigerinoides ruber and also major elements analyses in bulk sediment are presented. Our results show that significant changes in surface water properties occurred in W-Sicily characterized by a strong contrast in the seasonal hydrographic conditions during the S1 interval. This study proposes that the oceanographic changes in the eastern Mediterranean associated with the surface freshening promoted by the African monsoon likely triggered a restricted water exchange through the Strait of Sicily. This situation led to limited influence of the surface Atlantic waters into the studied area that favored the development of intense summer stratification and vertical winter mixing. This situation changed at about 7 kyr cal. BP when a decrease in the summer stratification probably reflected the influence of the eastward path of the surface Atlantic Waters. This situation would suggest a reinforcement of the water exchange through the Strait of Sicily that marked the end of the extreme conditions that prevailed in the eastern Mediterranean during the S1 formation. • Surface hydrographic reorganization related to the sapropel S1 is observed for the first time in the central Mediterranean. • The acquired data provide new evidences of a restricted sea surface water exchange during the S1 across the Strait of Sicily. • Paleo-hydrographic reconstructions are based on planktic foraminifera ecology, major element analyses and δ 18 O measurements.

Evaluating the impact of Mediterranean overflow on the large-scale Atlantic Ocean circulation using neodymium isotopic composition

Palaeo-oceanographical archives in the Mediterranean Sea indicate the occurrence of Sapropel depositions, which are characteristic of anoxic bottom conditions and of a stratified sea. The impact of such drastic changes in the thermohaline circulation of the Mediterranean Sea on the large-scale circulations in the North Atlantic basin is not fully understood. Here we evaluate the impact of a direct perturbation of the Mediterranean Sea circulation through different idealized simulations with freshwater release rates of 20, 50, 100, and 200 mSv (1 mSv = 103 m3/s), using a the IPSL-CM5A-LR global coupled atmosphere-ocean model, with simulations up to 1000 years long. The simulations are compared to available data for the Mediterranean Sea and the North Atlantic. The Neodymium isotopic composition (εNd) anomalies in the North Atlantic are clearly dependent on the rate of fresh water release in the Mediterranean Sea, and subsequent modification of Atlantic meridional overturning circulation (AMOC) intensity and subpolar gyre (SPG) extension. Based on comparison with seawater εNd records from Rockall Trough, we suggest that a limited release of freshwater around 50 mSv may be more representative of hydrological conditions of the Mediterranean Sea during the Sapropel S1 deposition. Our results indicate that a decrease in Mediterranean overflow was clearly a factor in the intensification of upper AMOC following the Early Holocene period, when a large eastward expansion of the subpolar gyre is simulated. Also, eastward expansion of the subpolar gyre due to freshwater release in the Mediterranean (e.g. Sapropel-like events) and its impact on the εNd signature highlight the fact that the modification of ocean circulation in the North Atlantic basin might be more complex than a straightforward change in the AMOC, since there may also be a large impact on barotropic circulation. The simulations also produce a significant decrease in the εNd values of the Iceland-Scotland Overflow Water (between −3.5 and − 1.3 εNd unit), which is consistent with changes observed during the S1 event in the reconstructions. Thus the 3-dimensional changes in baroclinic and barotropic circulations of the ocean, as reconstructed in some εNd records, are broadly consistent with the sensitivity simulations performed, and might thus be partly related to changes in Mediterranean overflow, which would have contributed to modification of the Atlantic Ocean circulation following a complex 3-dimensional pattern that is detailed in the paper.

Redox evolution and the development of oxygen minimum zones in the Eastern Mediterranean Levantine basin during the early Holocene

Oxygen Minimum Zones (OMZs) are expanding in modern oceans due to anthropogenically-driven climate and environmental change. In the Eastern Mediterranean Sea (EMS), OMZs developed in the early Holocene as a result of decreased intermediate water ventilation, increasing temperature, and increased Nile discharge and primary productivity. Here, we report benthic foraminiferal numbers (BFN) and species abundances, together with redox-sensitive trace metals (RSTM), and iron and phosphorus speciation from two sediment cores sampled at intermediate depths (1200 and 1430 m) from the SE Levantine shelf. The main aim of our study is to better understand the sequence of redox changes during sapropel S1 deposition caused by biogeochemical processes affecting the sapropel intermediate water mass. The use of benthic foraminifera indices (diversity and oxygen) together with iron speciation and RSTM (V, Mo and U) enables detailed description of the changing oxygen/redox status of the overlying water. Prior to sapropel S1 deposition at ∼10.2 ka BP, RSTM suggest that the overlying water was well oxygenated, but benthic foraminifera numbers (BFN) suggest that oxygen levels had already begun to decrease. There was then a pulse of increased export carbon from the enlarged Nile flood plume, as shown by increased BFN at the beginning of sapropel S1. Shortly after, RSTM and Fe-S systematics suggest that the water column transitioned from dysoxic to anoxic, non-sulfidic. Anoxic conditions then persisted at 1200 m depth, but RSTM and benthic foraminifera indices suggest that deeper waters at 1430 m were more likely dysoxic, until the 8.2 ka BP global cooling event. The benthic foraminifera and inorganic redox proxies then suggest a second period of anoxic, non-sulfidic conditions, with a gradual return to well ventilated waters at the end of sapropel deposition at ∼6 ka BP. There was enhanced burial of authigenic P throughout sapropel deposition, derived from the deposition and subsequent release of organic-P and iron bound-P during diagenesis. Phosphorus recycling from the sediment and in the overlying water column added reactive P to these mid-depth waters, a process which has the potential to result in a positive feedback in systems where such waters are upwelled into the photic zone. The past EMS thus represents a template which can be used to predict biogeochemical changes in settings that evolve towards anoxic, non sulfidic conditions, which may occur in some areas as modern climate and environment change causes the continued expansion of modern OMZs and hypoxic areas adjacent to modern major rivers.

High resolution paleo-environmental changes during the Sapropel 1 in the North Ionian Sea, central Mediterranean

High-resolution paleoceanographic reconstruction of surface water properties during the most recent Sapropel event (S1) has been carried out by means of quantitative analyses of planktonic foraminiferal assemblages, planktonic foraminiferal oxygen isotopes (δ18O) and XRF elemental data from a 655 m depth core recovered in the North Ionian Sea. The results show that the S1 interval presents two distinctive warm phases (S1a and S1b), separated by a cold interruption event (S1i). High resolution faunal and geochemical analyses allow to identify two sub-phases within S1a interval, the oldest one has similar characteristics to S1b interval while the youngest sub-phase has less stratified surface waters with relatively lower nutrient content. The high abundance of Globigerinoides ruber white variety opposite to the low percentages of Neogloboquadrina pachyderma during the pre-S1 phase suggests that the onset of surface waters stratification occurred prior to the beginning of Sapropel deposition, acting as a pre-conditioning phase. Paleo-productivity proxies indicate that the deposition of S1 initiated after an increase in nutrient content, potentially related to increased fluvial inputs. Based on the integrated ecological interpretation of our records we argue that S1a and S1b are characterized as warm, stratified and nutrient rich surface waters in the Ionian Sea, while proxies related to oxygen content indicate dysoxic deep waters linked to a combination of the high nutrient content and stratified water column. The S1 interruption phase is characterized by the entrance of colder waters that caused mixing of the stratified water column and re-ventilation of the deep dysoxic waters.

Reventilation Episodes During the Sapropel S1 Deposition in the Eastern Mediterranean Based on Holococcolith Preservation

AbstractOrganic‐rich layers (sapropels), preserved in eastern Mediterranean marine sediment records, represent pronounced perturbations to thermohaline circulation and environmental conditions in the basin, in response to enhanced African monsoon activity and subsequent massive freshwater discharge. During the most recent event, Sapropel S1 formed between 10.8 and 6.1 ka, when freshwater‐driven stratification caused seafloor anoxia below ~1,800‐m depth, as a result of both failure of deep water formation and enhanced productivity. Here we analyze coccolith assemblages from the open eastern Mediterranean that form a west‐east transect across the basin and provide insights on past environmental changes. We focus on holococcoliths, which are specifically produced by coccolithophores as part of their life cycle during the haploid phase. Since holococcolith calcification is characterized by nanocrystals highly susceptible to dissolution, we are testing their potential preservation under different bottom environmental conditions, including the effect of postdepositional oxidation. A comparison with benthic foraminifera assemblages in a core recovered close to Lybia reveals that holococcolith preservation is enhanced during seafloor reventilation and benthic foraminiferal repopulation in the middle to upper part of the record, before the actual sapropel termination. There are two such events of improved deep‐water oxygenation in the Aegean and Adriatic Seas at 8.2 and 7.4 ka. The latter episode marks the onset of the transition to restored circulation in the eastern Mediterranean Sea, due to resumption of deep‐water formation in the southern Aegean Sea and the conclusion of enhanced biogenic productivity.

Biomarker evidence for nitrogen-fixing cyanobacterial blooms in a brackish surface layer in the Nile River plume during sapropel deposition

Abstract Sapropels are organic-rich sediment layers deposited in the eastern Mediterranean Sea during precession minima, resulting from an increase in export productivity and/or preservation. Increased freshwater delivery from the African continent resulted in stratification, causing deepwater anoxia, while nutrient input stimulated productivity, presumably at the deep chlorophyll maximum. Previous studies have suggested that during sapropel deposition, nitrogen fixation was widespread in the highly stratified surface waters, and that cyanobacteria symbiotic with diatoms (diatom-diazotroph associations, DDAs) were responsible. Here we analyzed sapropel S5 sediments for heterocyst glycolipids (HGs) from three locations in the eastern Mediterranean. HG biomarkers can differentiate between those heterocystous cyanobacteria that are free living (found predominately in freshwater or brackish environments) and those that are from DDAs (found in marine settings). In our primary core, from a location which would have been influenced by the Nile River outflow, we detected a HG with a pentose (C5) head group specific for DDAs. However, HGs with a hexose (C6) head group, specific to free-living cyanobacteria, were present in substantially (up to 60×) higher concentration. These data suggest that at our study location, free-living cyanobacteria were the dominant diazotrophs, rather than DDAs. The C6 HGs increased substantially at the onset of sapropel S5 deposition, suggesting that substantial seasonal cyanobacterial blooms were associated with a brackish surface layer flowing from the Nile into the eastern Mediterranean. Two additional S5 sapropels were analyzed, one also from the Nile delta region and one from the region between Libya and southwestern Crete. Overall, comparison of the HG distribution in the three S5 sapropels provides evidence that all three locations were initially influenced by surface salinities that were sufficiently low to support free-living heterocystous cyanobacteria. While free-living heterocystous cyanobacteria continued to outnumber DDAs during sapropel deposition at the two Nile-influenced sites, DDAs, indicators of persistent marine salinities, were the dominant diazotrophs in the upper part of the sapropel at the more westerly site. These results indicate that N2 fixation by free-living cyanobacteria offers an important additional mechanism to stimulate productivity in regions with strong river discharge during sapropel deposition.

Dynamic surface-water alterations during sapropel S1 preserved in high-resolution shallow-water sediments of Taranto Gulf, central Mediterranean

The high sedimentation rate and shallow-water depositional depth of the sediments at site DP 30 in the Gulf of Taranto (central Mediterranean) provide unique biogenic signals that are usually not preserved in deeper records. This permits the reconstruction of long and short-term surface water modifications across the sapropel S1 period (10–6.5 ka), including calcareous nannofossil data at centennial-scale resolution.The peculiar patterns of key taxa allowed the recognition of distinct long to short-term surface water changes that provide valuable evidence on climate conditions leading to sapropel deposition. As a long-term pattern, S1 is marked by a distinct increase of Helicosphaera carteri, thought to reflect increased detrital input/land-derived nutrients in the surface water, related to a higher influence of the Western Adriatic Current. A concomitant pattern of increased black organic particles in the sediments supports a connection between organic matter preservation at the seafloor and nutrient availability in the surface water. Decreased salinity in the surface water, testified by an increase of Braurudosphaera bigelowii, develops from 9.2 up to 6.5 ka.At higher frequencies, additional changes occurred in surface-subsurface waters during the formation of S1 at shallow depths. Starting from 11 ka, the relative abundance pattern of Florisphaera profunda indicates a gradual nutrient enrichment in the subsurface waters and the development of a deep chlorophyll maximum (DCM), which becomes well established at the onset of sapropel S1, between 10 and 9 ka. A further change in the plankton ecosystem occurs between 9 and 8.2 ka, when reduced coccolithophore production is concomitant with diatom occurrence and enhanced Br/Ti level, implying a productivity increase. From 8 to 6.5 ka, coccolithophore productivity is re-established in relation to a reduction both in surface-water stratification and turbidity/nutrient availability, characterizing the final phase of sapropel formation. The turbidity in the surface waters was briefly reduced at 8.2 and 7.3 ka, when also oxygen conditions at the seafloor improved. A centennial variability in turbidity/low light conditions in surface waters is evident across S1 and is illustrated by the abundance patterns of H. carteri and Gladiolithus flabellatus. The occurrence of ascidian spicules also represents a distinct biotic signal across the S1 layer, indicating an increase of shallow-water biogenic carbonate or an aragonitic near-coastal source. Surface-water modifications at longer and shorter time scales, accompanying the deposition of sapropel S1 in the Gulf of Taranto, occurred nearly simultaneously and with a magnitude that is comparable to changes observed in S1 deposits in the central and eastern Mediterranean basin and slope settings.

Multiproxy ecosystem response of abrupt Holocene climatic changes in the northeastern Mediterranean sedimentary archive and hydrologic regime

AbstractAspects of paleoclimatic and paleoceanographic evolution of the north Aegean Sea through the Holocene are revealed by the study of quantitative variations in planktonic foraminiferal, pteropodal, and palynomorph assemblages; the isotopic composition of planktonic foraminifera; and hydrographic-related indices, extracted from two high-sedimentation rate cores from the North Aegean Trough. Focusing on the last ~10 cal ka BP, the current Holocene subdivision (Greenlandian, Northgrippian, and Meghalayan) confirms the traditional understanding of an evolution from wetter (Greenlandian) to gradually drier (Northgrippian and Meghalayan) climatic conditions and further highlights the role of changing seasonality during this time. The most warm and humid phase corresponds to the time of the sapropel S1 deposition (9.6–6.1 cal ka BP). The Holocene climatic instability of the study area is further supported by six episodes of brief cooling (North Aegean cooling; NAEGC6–NAEGC1) centered at 9.30, 8.05, 7.05, 4.55, 3.55, and 2.05 cal ka BP, reflected by significant faunal changes and oxygen isotope enrichments. These cold/arid events are coeval with equivalent cooling events that have been described in different basins of the Mediterranean Sea, while signal similarities with equivalent changes in the intensity of the Siberian high suggest a climatic link between the studied area and the high-latitude areas.

Impact of latest-glacial to Holocene sea-level oscillations on central Aegean shelf ecosystems: A benthic foraminiferal palaeoenvironmental assessment of South Evoikos Gulf, Greece

Abstract A high-resolution palaeoenvironmental investigation was conducted on the benthic foraminiferal record of two continuous sediment cores (DEH 5, DEH 1) in order to reconstruct the latest-Pleistocene to Holocene history of the South Evoikos Gulf (central Aegean Sea). This shallow (max. depth ~75 m), semi-enclosed continental shelf setting has been heavily affected by the latest-glacial to modern-interglacial relative sea-level and climate oscillations that strongly affected the benthic foraminiferal community. The identified meiofaunal biofacies associated with the chronological framework allowed the recognition of four main time phases in the palaeoenvironmental and palaeobathymetric evolution of the gulf: (i) During the latest glacial-interglacial transition (>15.17 ka BP), highly-stressed confined shallow lagoonal conditions correspond to the Last Glacial Maximum (LGM) sea-level lowstand. During that time, a natural barrier located at the mouth of the gulf effectively prevented communication with the open Aegean Sea and amplified the contribution of the local aquifers. (ii) The early post-LGM sea-level rise was initially marked by a weak marine connection, dominated by a pronounced seaward river progradation of local aquifers. However, immediately after (~14.7 ka BP), an enhanced rate of sea-level rise, synchronous with the Meltwater Pulse 1A (MWP-1A), is observed. At the end of the Pleistocene, the sedimentation rates decreased dramatically, as a result of the Younger Dryas (YD) cooling event. (iii) The Holocene sea-level rise was characterized by an extremely abrupt marine transgression (~11.29 ka BP), during which the sea-level in the Aegean Sea reached 55 m below present sea level (b.p.s.l) and forcedly overcame the barrier that previously separated the gulf from the open sea. At ~8.67–7.77 ka BP, an upper circalittoral eutrophic environment emerged, which coincided with the coeval deposition of the sapropel S1 in the Eastern Mediterranean Sea. Immediately after, re-ventilation conditions were reinstated. (iv) The Holocene sea-level high-stand is characterized by upper circalittoral high-energy and vegetated conditions on the sea floor. Regional index terms Eastern Mediterranean; Central Aegean Sea; Greece; South Evoikos Gulf.

Eastern Mediterranean Deep Water Formation During Sapropel S1: A Reconstruction Using Geochemical Records Along a Bathymetric Transect in the Adriatic Outflow Region

AbstractEastern Mediterranean thermohaline circulation is directly influenced by middle‐ and low‐latitude climate systems. The dramatic paleoclimate changes during the last African Humid Period (~10–6 ka BP) were captured in Mediterranean sediments as the distinctly organic‐rich unit sapropel S1. Here, deepwater formation variability during S1 deposition is reconstructed. We use geochemical records of three cores along a bathymetric transect (775‐, 1,359‐, and 1,908‐m water depths), at the transition between the Adriatic DW‐ formation area and the Eastern Mediterranean. In all three cores, sedimentation rates are distinctly higher during S1, corresponding with enhanced runoff emanating from the Adriatic hinterland. Hence, major runoff did not only come from southern but also from northern borderlands in this period. During sapropel formation, enhanced levels of primary productivity occurred in the surface waters and oxygen‐depleted conditions in the bottom waters for all sites. Conditions for sediment and bottom‐water below ~1.4 km water depth were sulfidic throughout S1, but for intermediate depth (775 m) were anoxic only during the first part (S1a). Bottom‐water oxygenation interrupted S1 formation at water depths down to ~1.4 km, during two brief episodes, at 8.2 and 7.4 cal. ka BP. From the 7.4 cal. ka BP ventilation onward, the transition to more oxygenated bottom‐water conditions was more progressive for the intermediate water depth site (775 m) than for the deeper sites. Conditions remained fully oxic for all water depths following the S1‐MarkerBed ventilation event. Possibly, the onset of continuously oxic conditions started slightly earlier at intermediate depth (775 m; 6.6 ± 0.3 cal. ka BP) than at greater depths (1,359 m, 1,908 m; 6.0 ± 0.3 cal. ka BP).

The nanostructure and microstructure of SiC surface layers deposited by MWCVD and ECRCVD

Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) have been used to investigate ex-situ the surface topography of SiC layers deposited on Si(100) by Microwave Chemical Vapour Deposition (MWCVD) -S1,S2 layers and Electron Cyclotron Resonance Chemical Vapor Deposition (ECRCVD) – layers S3,S4, using silane, methane, and hydrogen. The effects of sample temperature and gas flow on the nanostructure and microstructure have been investigated. The nanostructure was described by three-dimensional surface roughness analysis based on digital image processing, which gives a tool to quantify different aspects of surface features. A total of 13 different numerical parameters used to describe the surface topography were used. The scanning electron image (SEM) of the microstructure of layers S1, S2, and S4 was similar, however, layer S3 was completely different; appearing like grains. Nonetheless, it can be seen that no grain boundary structure is present in the AFM images.

Large-scale response of the Eastern Mediterranean thermohaline circulation to African monsoon intensification during sapropel S1 formation

The formation of Eastern Mediterranean sapropels has periodically occurred during intensification of northern hemisphere monsoon precipitation over North Africa. However, the large-scale response of the Eastern Mediterranean thermohaline circulation during these monsoon-fuelled freshening episodes is poorly constrained. Here, we investigate the formation of the youngest sapropel (S1) along an across-slope transect in the Adriatic Sea. Foraminifera-based oxygen index, redox-sensitive elements and biogeochemical parameters reveal – for the first time – that the Adriatic S1 was synchronous with the deposition of south-eastern Mediterranean S1 beds. Proxies of paleo thermohaline currents indicate that the bottom-hugging North Adriatic Dense Water (NAdDW) suddenly decreased at the sapropel onset simultaneously with the maximum freshening of the Levantine Sea during the African Humid Period. We conclude that the lack of the “salty” Levantine Intermediate Water hampered the preconditioning of the northern Adriatic waters necessary for the NAdDW formation prior to the winter cooling. Consequently, a weak NAdDW limited in turn the Eastern Mediterranean Deep Water (EMDWAdriatic) formation with important consequences for the ventilation of the Ionian basin as well. Our results highlight the importance of the Adriatic for the deep water ventilation and the interdependence among the major eastern Mediterranean water masses whose destabilization exerted first-order control on S1 deposition.

Hydrological variations of the intermediate water masses of the western Mediterranean Sea during the past 20 ka inferred from neodymium isotopic composition in foraminifera and cold-water corals

Abstract. We present the neodymium isotopic composition (εNd) of mixed planktonic foraminifera species from a sediment core collected at 622 m water depth in the Balearic Sea, as well as εNd of scleractinian cold-water corals (CWC; Madrepora oculata, Lophelia pertusa) retrieved between 280 and 442 m water depth in the Alboran Sea and at 414 m depth in the southern Sardinian continental margin. The aim is to constrain hydrological variations at intermediate depths in the western Mediterranean Sea during the last 20 kyr. Planktonic (Globigerina bulloides) and benthic (Cibicidoides pachyderma) foraminifera from the Balearic Sea were also analyzed for stable oxygen (δ18O) and carbon (δ13C) isotopes. The foraminiferal and coral εNd values from the Balearic and Alboran seas are comparable over the last ∼ 13 kyr, with mean values of −8.94 ± 0.26 (1σ; n = 24) and −8.91 ± 0.18 (1σ; n = 25), respectively. Before 13 ka BP, the foraminiferal εNd values are slightly lower (−9.28 ± 0.15) and tend to reflect higher mixing between intermediate and deep waters, which are characterized by more unradiogenic εNd values. The slight εNd increase after 13 ka BP is associated with a decoupling in the benthic foraminiferal δ13C composition between intermediate and deeper depths, which started at ∼ 16 ka BP. This suggests an earlier stratification of the water masses and a subsequent reduced contribution of unradiogenic εNd from deep waters. The CWC from the Sardinia Channel show a much larger scatter of εNd values, from −8.66 ± 0.30 to −5.99 ± 0.50, and a lower average (−7.31 ± 0.73; n = 19) compared to the CWC and foraminifera from the Alboran and Balearic seas, indicative of intermediate waters sourced from the Levantine basin. At the time of sapropel S1 deposition (10.2 to 6.4 ka), the εNd values of the Sardinian CWC become more unradiogenic (−8.38 ± 0.47; n = 3 at ∼ 8.7 ka BP), suggesting a significant contribution of intermediate waters originated from the western basin. We propose that western Mediterranean intermediate waters replaced the Levantine Intermediate Water (LIW), and thus there was a strong reduction of the LIW during the mid-sapropel ( ∼ 8.7 ka BP). This observation supports a notable change of Mediterranean circulation pattern centered on sapropel S1 that needs further investigation to be confirmed.

Preliminary results of high resolution paleoceanography and paleoclimatology during sapropel S1 deposition (South Limnos Basin, North Aegean Sea).

The paleoenviromental conditions during the depositional interval of sapropel S1 in the northeastern Aegean (gravity core M-4, length 2.53 m; south Limnos basin) are studied based on quantitative micropaleontological (benthic and planktonic foraminifera) and geochemical (OC, δ13Corg) analyses. Special feature of core M-4 is the thickness of S1 layer (96 cm). Our study points that sapropelic layer S1a has been deposited in more dysoxic and warmer conditions in respect to S1b. Both primary productivity and preservation of organic material are more intense during the lower part of S1. An interruption of the sapropelic conditions at 8.0 Ka BP which is mainly characterized by the increase of agglutinated foraminiferal forms confirms both higher oxygen bottom conditions and freshwater input.

Eastern-Mediterranean ventilation variability during sapropel S1 formation, evaluated at two sites influenced by deep-water formation from Adriatic and Aegean Seas

Present-day bottom-water ventilation in the Eastern Mediterranean basin occurs through deep-water convection originating from the two marginal basins, i.e. Adriatic and Aegean Seas. In the paleo record, long periods of enhanced deep-water formation have been alternating with shorter periods of reduced deep-water formation. The latter is related mainly to low-latitude humid climate conditions and the enhanced deposition and preservation of organic-rich sediment units (sapropels). This study focuses on sedimentary archives of the most-recent sapropel S1, retrieved from two sites under the direct influence of the two deep-water formation areas. Restricted oxygen conditions have developed rapidly at the beginning of S1 deposition in the Adriatic site, but bottom-water conditions have not persistently remained anoxic during the full interval of sapropel deposition. In fact, the variability in intensity and persistence of sedimentary redox conditions at the two deep-water formation sites is shown to be related to brief episodes of climate cooling. In the Adriatic site, sapropel deposition appears to have been interrupted twice. The 8.2 ka event, only recovered at the Adria site, is characterized by gradually increasing suboxic to possibly intermittently oxic conditions and decreasing Corg fluxes, followed by an abrupt re-establishment of anoxic conditions. Another important event that disrupted sapropel S1 formation, has taken place at ca. 7.4 cal ka BP. The latter event has been recovered at both sites. In the Adriatic site it is followed by a period of sedimentary conditions that gradually change from suboxic to more permanently oxic, as deduced from the Mn/Al pattern. Using the same proxy for suboxic/oxic sedimentary redox conditions, we observe that conditions in the Aegean Sea site shift to more permanently oxic from the 7.4 ka event onwards. However, at both sites the accumulation and preservation of enhanced amounts of organic matter have continued under these suboxic to intermittently oxic sedimentary conditions. It seems thus, that after 7.4 cal ka BP sapropel-like surface or deep-chlorophyll-maximum conditions including enhanced productivity continued, whereas bottom-water conditions were at least intermittently oxic. The latter is related to decreasing precipitation, i.e. run-off, and thus a progressive development and deepening of deep-water formation. The shallower Aegean site, would be affected earlier by such deepening ventilation than the slightly deeper Adriatic site. Finally, termination of sapropel S1 formation as deduced from diminished organic matter contents and Ba/Al, appears to have occurred almost simultaneously in the two areas, namely at 6.6 ± 0.3 and 6.3 ± 0.5 cal ka BP in Adriatic and Aegean sites, respectively.

Holocene Climatic Optimum centennial-scale paleoceanography in the NE Aegean (Mediterranean Sea)

Combined micropaleontological and geochemical analyses of the high-sedimentation gravity core M-4G provided new centennial-scale paleoceanographic data for sapropel S1 deposition in the NE Aegean Sea during the Holocene Climatic Optimum. Sapropel layer S1a (10.2–8.0 ka) was deposited in dysoxic to oxic bottom waters characterized by a high abundance of benthic foraminiferal species tolerating surface sediment and/or pore water oxygen depletion (e.g., Chilostomella mediterranensis, Globobulimina affinis), and the presence of Uvigerina mediterranea, which thrives in oxic mesotrophic-eutrophic environments. Preservation of organic matter (OM) is inferred based on high organic carbon as well as loliolide and isololiolide contents, while the biomarker record and the abundances of eutrophic planktonic foraminifera document enhanced productivity. High inputs of terrigenous OM are attributed to north Aegean borderland riverine inputs. Both alkenone-based sea surface temperatures (SSTs) and δO18G. bulloides records indicate cooling at 8.2 ka (S1a) and ~7.8 ka (S1 interruption). Sapropelic layer S1b (7.7–6.4 ka) is characterized by rather oxic conditions; abundances of foraminiferal species tolerant to oxygen depletion are very low compared with the U. mediterranea rise. Strongly fluctuating SSTs demonstrate repeated cooling and associated dense water formation, with a major event at 7.4 ka followed by cold spells at 7.0, 6.8, and 6.5 ka. The prominent rise of the carbon preference index within the S1b layer indicates the delivery of less degraded terrestrial OM. The increase of algal biomarkers, labile OM-feeding foraminifera and eutrophic planktonic species pinpoints an enhanced in situ marine productivity, promoted by more efficient vertical convection due to repeated cold events. The associated contributions of labile marine OM along with fresher terrestrial OM inputs after ~7.7 ka imply sources alternative/additional to the north Aegean riverine borderland sources for the influx of organic matter in the south Limnos Basin, plausibly related to the inflow of highly productive Marmara/Black Sea waters.