The “glacial” sapropel S6 (172 ka; MIS 6): A multiproxy approach to solve a Mediterranean “cold case”

Sequences of dark, organic-rich sediment layers (sapropels) exist throughout the Neogene of the Mediterranean Sea sedimentary record. While the mechanisms behind their cyclical deposition are not entirely understood, they have been found to correspond with precession minima (Northern Hemisphere [NH] insolation maxima). This causes NH summer monsoon to shift northward and intensify, which increases precipitation over North Africa and alters Mediterranean freshwater budget, leading to restricted bottom-water ventilation and anoxia. Most Mediterranean sapropels were deposited during interglacial periods, but sapropel S6 formed during the penultimate glaciation of Marine Isotopic Stage 6 (MIS 6; 190–130 ka), during which the Eurasian ice sheet extended to its maximum size of the Quaternary. Eurasian ice-sheet melting may have provided an additional input of freshwater to the Mediterranean during S6 deposition. To test this hypothesis, we present a multiproxy paleoecological (planktic foraminifera, calcareous nannofossils, pollen, dinocysts) and geochemical (foraminiferal δ18O) study of S6 from the Ionian Sea. We confirm that S6 deposition resulted from an interaction of two different mechanisms of freshwater input to the Mediterranean, in which: (1) local ice-sheet meltwater discharge preconditioned the basin for stratification; and (2) increased monsoon activity over North Africa caused intense precipitation and river runoff that exacerbated water-column stratification. Our results provide new evidence for the prevalence of mild/temperate and humid conditions during S6 deposition, dispelling the notion that this “glacial” sapropel formed under cold and dry conditions and we document signals of warm (interstadial) and cold (stadial) conditions within S6 in the eastern Mediterranean basin

Dinoflagellate cyst and pollen assemblages as tracers for marine productivity and river input in the northern Gulf of Mexico

Both marine dinoflagellate cysts and terrestrially derived pollen and spores are abundant in coastal sediments close to river mouths, making sediment records from such settings ideal to simultaneously study land– ocean climate interactions, marine productivity patterns and freshwater input over time. However, few studies consider the combined calibration of these palynological proxies in modern coastal sediments offshore from rivers, which is needed to strengthen the interpretation of paleoreconstructions. Here, we analyze the palynological content of marine surface sediments along land–sea transects off the Mississippi and Atchafalaya river mouths in the northern Gulf of Mexico (GoM) and test three palynological indices which are often employed in the paleo-domain: (i) the abundance of cysts of heterotrophic and autotrophic dinoflagellates (dinocysts) as a tracer for primary productivity (H /A ratio) and (ii) the ratio between non-bisaccate pollen and bisaccates (P /B) as well as (iii) the ratio between pollen (excluding bisaccates) and dinocysts (P /D), which are both tracers for river input and distance to the coast. Our results show that dinoflagellate cysts are most abundant on the shelf, where heterotrophic dinocyst species dominate coastal assemblages in reach of the river plume, while autotrophic taxa are more present in the oligotrophic open ocean. This is clearly reflected in decreasing H/A values further offshore. Individual dinocyst taxa also seem to inhabit specific niches along an onshore–offshore transect, linked to nutrient availability and proximity to the turbid river plume. The highest pollen concentrations are found close to the Mississippi river mouth and mostly represent a mixture of local coastal and upstream vegetation, whereas bisaccate pollen was most abundant further offshore of the Mississippi river. Multivariate redundancy analysis (RDA) performed on both pollen and dinocyst assemblages, a set of environmental parameters, and the three palynological ratios showed that net primary productivity was the most important variable influencing the dinocyst assemblages, likely as the result of nutrient input. Additionally, the RDA confirmed that the H/A ratio indeed seems to track primary productivity, while the P /B ratio results in a robust indicator for distance to the coast, and the P /D ratio better reflects river input. Together, our data confirm and further specify the suitability of these three palynological ratios in river-dominated coastal margins as proxies for (past) marine productivity and distance to the coast and river

The “glacial” sapropel S6 (172 ka; MIS 6): A multiproxy approach to solve a Mediterranean “cold case”

Sequences of dark, organic-rich sediment layers (sapropels) exist throughout the Neogene of the Mediterranean Sea sedimentary record. While the mechanisms behind their cyclical deposition are not entirely understood, they have been found to correspond with precession minima (Northern Hemisphere [NH] insolation maxima). This causes NH summer monsoon to shift northward and intensify, which increases precipitation over North Africa and alters Mediterranean freshwater budget, leading to restricted bottom-water ventilation and anoxia. Most Mediterranean sapropels were deposited during interglacial periods, but sapropel S6 formed during the penultimate glaciation of Marine Isotopic Stage 6 (MIS 6; 190–130 ka), during which the Eurasian ice sheet extended to its maximum size of the Quaternary. Eurasian ice-sheet melting may have provided an additional input of freshwater to the Mediterranean during S6 deposition. To test this hypothesis, we present a multiproxy paleoecological (planktic foraminifera, calcareous nannofossils, pollen, dinocysts) and geochemical (foraminiferal δ18O) study of S6 from the Ionian Sea. We confirm that S6 deposition resulted from an interaction of two different mechanisms of freshwater input to the Mediterranean, in which: (1) local ice-sheet meltwater discharge preconditioned the basin for stratification; and (2) increased monsoon activity over North Africa caused intense precipitation and river runoff that exacerbated water-column stratification. Our results provide new evidence for the prevalence of mild/temperate and humid conditions during S6 deposition, dispelling the notion that this “glacial” sapropel formed under cold and dry conditions and we document signals of warm (interstadial) and cold (stadial) conditions within S6 in the eastern Mediterranean basi

A new quantitative approach to identify reworking in Eocene to Miocene pollen records from offshore Antarctica using red fluorescence and digital imaging

Antarctic palaeoclimate evolution and vegetation history after the formation of a continent-scale cryosphere at the Eocene–Oligocene boundary, 33.9 million years ago, has remained a matter of controversy. In particular, the reconstruction of terrestrial climate and vegetation has been strongly hampered by uncertainties in unambiguously identifying non-reworked as opposed to reworked sporomorphs that have been transported into Antarctic marine sedimentary records by waxing and waning ice sheets. Whereas reworked sporomorph grains over longer non-successive geological timescales are easily identifiable within younger sporomorph assemblages (e.g. Permian sporomorphs in Pliocene sediments), distinguishing non-reworked from reworked material in palynological assemblages over successive geological time periods (e.g. Eocene sporomorphs in Oligocene sediments) has remained problematic. This study presents a new quantitative approach to identifying non-reworked pollen assemblages in marine sediment cores from circum-Antarctic waters. We measured the fluorescence colour signature, including red, green, and blue fluorescence; brightness; intensity; and saturation values of selected pollen and spore taxa from Eocene, Oligocene, and Miocene sediments from the Wilkes Land margin Site U1356 (East Antarctica) recovered during Integrated Ocean Drilling Program (IODP) Expedition 318. Our study identified statistically significant differences in red-fluorescence values of non-reworked sporomorph taxa against age. We conclude that red fluorescence is a reliable parameter for identifying the presence of non-reworked pollen and spores in Antarctic marine sediment records from the circum-Antarctic realm that are influenced by glaciation and extensive reworking. Our study provides a new tool to accurately reconstruct Cenozoic terrestrial climate change on Antarctica using fossil pollen and spores

The dispersal of fluvially discharged and marine, shelf-produced particulate organic matter in the northern Gulf of Mexico

Rivers play a key role in the global carbon cycle by transporting terrestrial organic matter (TerrOM) from land to the ocean. Upon burial in marine sediments, this TerrOM may be a significant long-term carbon sink, depending on its composition and properties. However, much remains unknown about the dispersal of different types of TerrOM in the marine realm upon fluvial discharge since the commonly used bulk organic matter (OM) parameters do not reach the required level of source- and process-specific information. Here, we analyzed bulk OM properties, lipid biomarkers (long-chain n-alkanes, sterols, long-chain diols, alkenones, branched and isoprenoid glycerol dialkyl glycerol tetraethers (brGDGTs and isoGDGTs)), pollen, and dinoflagellate cysts in marine surface sediments along two transects offshore the Mississippi-Atchafalaya River (MAR) system, as well as one along the 20 m isobath in the direction of the river plume. We use these biomarkers and palynological proxies to identify the dispersal patterns of soil-microbial organic matter (SMOM), fluvial, higher plant, and marine-produced OM in the coastal sediments of the northern Gulf of Mexico (GoM). The Branched and Isoprenoid Tetraether (BIT) index and the relative abundance of C32 1,15-diols indicative for freshwater production show high contributions of SMOM and fluvial OM near the Mississippi River (MR) mouth (BIT Combining double low line 0.6, FC321,15 > 50 %), which rapidly decrease further away from the river mouth (BIT < 0.1, FC321,15 < 20 %). In contrast, concentrations of long-chain n-alkanes and pollen grains do not show this stark decrease along the path of transport, and especially n-alkanes are also found in sediments in deeper waters. Proxy indicators show that marine productivity is highest close to shore and reveal that marine producers (diatoms, dinoflagellates, coccolithophores) have different spatial distributions, indicating their preferred niches. Close to the coast, where food supply is high and waters are turbid, cysts of heterotrophic dinoflagellates dominate the assemblages. The dominance of heterotrophic taxa in shelf waters in combination with the rapid decrease in the relative contribution of TerrOM towards the deeper ocean suggest that TerrOM input may trigger a priming effect that results in its rapid decomposition upon discharge. In the open ocean far away from the river plume, autotrophic dinoflagellates dominate the assemblages, indicating more oligotrophic conditions. Our combined lipid biomarker and palynology approach reveals that different types of TerrOM have distinct dispersal patterns, suggesting that the initial composition of this particulate OM influences the burial efficiency of TerrOM on the continental margin

Paratethys pacing of the Messinian Salinity Crisis:Low salinity waters contributing to gypsum precipitation?

During the so-called Messinian Salinity Crisis (MSC: 5.97-5.33 Myr ago), reduced exchange with the Atlantic Ocean caused the Mediterranean to develop into a “saline giant” wherein ∼1 million km3 of evaporites (gypsum and halite) were deposited. Despite decades of research it is still poorly understood exactly how and where in the water column these evaporites formed. Gypsum formation commonly requires enhanced dry conditions (evaporation exceeding precipitation), but recent studies also suggested major freshwater inputs into the Mediterranean during MSC-gypsum formation. Here we use strontium isotope ratios of ostracods to show that low-saline water from the Paratethys Seas actually contributed to the precipitation of Mediterranean evaporites. This apparent paradox urges for an alternative mechanism underlying gypsum precipitation. We propose that Paratethys inflow would enhance stratification in the Mediterranean and result in a low-salinity surface-water layer with high Ca/Cl and SO4/Cl ratios. We show that evaporation of this surface water can become saturated in gypsum at a salinity of ∼40, in line with salinities reported from fluid inclusions in MSC evaporites

Whose Ocean? Exploring multidisciplinary perspectives towards ocean sustainability and implications for the un(der)represented

The ocean's significance encompasses crucial ecosystem services including climate regulation, oxygen production and food supply. The ocean is also a major player in the global economy. However, human activities continue to harm the ocean, jeopardising these vital functions. In July 2022, the United Nations Ocean Conference adopted a political declaration entitled "Our ocean, our future, our responsibility," emphasising the need for sustainable ocean management and protection. However, an important initial question arises: who are the “Our”? or, rephrased “Whose ocean” is it? This study presents first answers to this question, based on interviews with ocean professionals from diverse backgrounds. Their responses showcased the complexity of the issue, with differing opinions on ocean “ownership” and “control”. Despite the diversity of perspectives, a shared emphasis emerged: shifting from profit-driven decision-making to prioritising marine ecosystem health. Proposed approaches to build a sustainable relationship between people and the ocean include promoting ocean literacy and marine research and ensuring global accountability. These voices offered valuable insights towards ocean sustainability, guiding future academic, educational and policy-making efforts

Whose Ocean? Exploring multidisciplinary perspectives towards ocean sustainability and implications for the un(der)represented

The ocean's significance encompasses crucial ecosystem services including climate regulation, oxygen production and food supply. The ocean is also a major player in the global economy. However, human activities continue to harm the ocean, jeopardising these vital functions. In July 2022, the United Nations Ocean Conference adopted a political declaration entitled &quot;Our ocean, our future, our responsibility,&quot; emphasising the need for sustainable ocean management and protection. However, an important initial question arises: who are the “Our”? or, rephrased “Whose ocean” is it? This study presents first answers to this question, based on interviews with ocean professionals from diverse backgrounds. Their responses showcased the complexity of the issue, with differing opinions on ocean “ownership” and “control”. Despite the diversity of perspectives, a shared emphasis emerged: shifting from profit-driven decision-making to prioritising marine ecosystem health. Proposed approaches to build a sustainable relationship between people and the ocean include promoting ocean literacy and marine research and ensuring global accountability. These voices offered valuable insights towards ocean sustainability, guiding future academic, educational and policy-making efforts