United States

We present here data on changes in Mg/Ca derived paleotemperatures from the southern Atlantic Site 1263 across the Early Eocene Climatic Optimum (EECO; ~53-49 Ma). The EECO, that records the peak of Cenozoic warmth, and possibly of the CO2 pressure (e.g., Zachos et al., 2001, Sciences; Anagnostou et al. 2016, Nature Inglis et al., 2020 Clim. Past), induced climatic and paleoceaographic changes that significantly impacted planktic foraminferal assemblages. The main change, recorded form several sites (Atlantic and Pacific Oceans, Tethys) is the permanent and markedly declined in abundance and diversity of the mixed-layer dwelling symbiotic bearing genus Morozovella coupled with the increase of genus Acarinina (Luciani et al., 2016; Luciani et al. 2017a; Luciani et al. 2017b; D’Onofrio et al., 2020). This decline occurred at the EECO beginning, close to the carbonate isotope excursion known as ‘J event’ (~53 Ma). In addition, a reduction in morozovellid test-size occurred across EECO as recorded from Atlantic ODP sites 1051, 1258 and 1263. Several potential stressors may explain both the reduced size and the permanent morozovellid decline. These include algal photosymbiont inhibition (bleaching), increase in temperature, decrease in pH and/or calcite saturation state. Even though a bleaching test at Site 1051 revealed only a transient reduction of algal-symbiont relationships just after the morozovellid abundance decline (Luciani et al., 2017a), a general reduction in δ13C gradient (~ 0.5 ‰) between morozovellids and the thermocline dweller Subbotina spp. through the EECO is recorded from the Atlantic sites. This suggests that the former group became less reliant on photosymbionts and/or may have moved to slightly deeper depth in the mixed-layer. Even though a link with the EECO perturbation is evident, the driving causes of the recorded modifications are still unknown. Detailed paleotemperatures across the observed changes were lacking. We therefore obtained Mg/Ca derived paleotemperatures from Site 1263 through Laser Ablation, a method widely applied. Preliminary Mg/Ca data reveal that Morozovella crater and M. subbotinae record a major warming across EECO greater than that of Acarinina coalingensis and A. soldadoensis.Although the exact causes of photosymbiont bleaching can be manifold, increased temperature is considered a primary cause of bleaching in present tropical larger benthic foraminifera (e.g., Hallock, 2000). The higher rise in temperature recorded by morozovellid may explain the reduced symbiotic relationship and suggests one possible reason for their reduction in abundance and size, even though other potential stressor such as pH decrease should be explored. We record from Site 1263 also a temperature increase at the thermocline that may be one of the causes explaining the drop in abundance of subbotinids and chiloguembelinids at the EECO. 

SGI 2021

Mg/Ca Paleotempertures changes at the demise of plankic foraminiferal genus Morozovella across the Early Eocene Climatic Optimum  (Site 1263, Southern Atlantic Ocean)

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The presence of different types of glacial and sub-glacial features near the shelf edge indicates that the Western Antarctic Ice Sheet (WAIS) was grounded near the shelf break in many sectors of the Eastern Ross Sea (ERS) during the Last Glacial Maximum (LGM) (Shipp et al., 1999; Mosola & Anderson, 2006; Halberstadt et al., 2016; Anderson et al., 2018; Gales et al., 2021). However, mechanisms and timing of the post-LGM retreat are still debated. Studying sediments collected along the continental slope could help to reconstruct the LGM and post-LGM history of the ice sheet. This study focused on the analysis of three box cores collected east of the Hillary Canyon, which carves the ERS continental slope and is connected to the Glomar Challenger and Pennell Troughs on the shelf, south-east to the Iselin Bank. Multidisciplinary analysis (grain size, organic matter, biogenic silica and water content, magnetic susceptibility and chemical composition (XRF core scanner)) were conducted in order to reconstruct the sedimentary dynamics. Three samples for each box core were dated by using organic matter (14C AIO dates). Here, we present the results of these analysis. These box cores were collected during the XXIX PNRA (National Antarctic Research Program) expedition (2013-2014) and studied in the frame of the ROSSLOPE II (2013/AN2.01) PNRA project and the STREAM Project (Late Quaternary evolution of the ocean-ice sheet interactions: the record from the Ross Sea continental margin, Antarctica; period 2019-2021). The latter is funded by the Italian Ministry of Foreign Affairs and International Cooperation and the National Research Foundation of Korea.

Holocene sedimentary dynamics by multidisciplinary analysis of three box cores collected East to the Hillary Canyon (Eastern Ross Sea, Antarctica)

During the Last Glacial Maximum (LGM, 21’000 yrs BP) the Antarctic ice sheet was extended over the continental shelves. In the Ross Sea sector, only a restricted portion of the continental shelf was ice free, restricting main coastal circulation to the continental slope. How this impacted on the oceanic circulation in the open ocean and on the basal melting at the grounding line of the Antarctic ice sheet remains unknown. Here we present the implementation of the ocean model MITgcm on a regional grid of the Ross Sea at 5 km resolution. The numerical model includes the sub ice-shelf cavity and interactive sea-ice, while tides are not accounted for. This implementation was used to run a transient control simulation over 1993-2018 forced by state-of-the-art atmospheric and ocean reanalyses. As for the LGM, the model was spun up with steady-state climatic conditions and run transiently for 20 years. Atmospheric and oceanic forcing were taken from a global climate transient simulation (TraCE-21ka, Liu et al., 2009). The comparison of model output with oceanographic in situ data, demonstrates that it is able to simulate the main features of the oceanic circulation: the warm Circumpolar Deep Water (CDW) is advected by the southern branch of the Ross Sea Gyre towards the shelf-break, where it crosses the Antarctic Slope Front (ASF) and intrudes onto the shelf where it mixes with ambient waters (mCDW). In the Western shelf, production of High Salinity Shelf Water (HSSW) occurs by wind-induced brine rejection in winter months. The HSSW and mCDW are advected below the Ross ice shelf at different depths, the first at the grounding line, the second at the front, therefore impacting in different ways on ice shelf dynamics. A preliminary comparison of the present-day and LGM runs shows that the simulated LGM circulation was totally different. Almost all the shelf was covered by grounded ice, except for the three main troughs in the Western side: Drygalski, Joides, Pennel (Halberstadt et al., 2016), where sub-ice shelf cavities presumably were developed. The CDW in the open ocean was ~2 °C cooler than the present, and ~2 PSU saltier. The ASF was largely missing, due to the weak density contrast between shelf and open ocean waters. Most oceanic processes occurring at present on the continental shelf, such as coastal polynyas, HSSW production and dense water formation still took place, but were confined to the continental slope, with increased intensity due to the more extreme climatic conditions at the LGM.

The Ross Sea sector oceanography: glacial vs present day

Ostracods, calcareous bivalve microcrustaceans, are the only metazoan organisms commonly preserved as microfossils in deep sea sediments in sufficient quantities for detailed paleoceanographic studies. Detailed investigations of modern and recent regional distribution of ostracod species from shelf, slope, bathyal, and abyssal environments indicate that their distribution is strongly influenced by the physico-chemical characteristics of water masses (e.g., temperature, salinity, nutrients, dissolved oxygen, etc.), as well as substrate type and food supply. The Quaternary Antarctic ostracod fauna is mostly composed of extant species. This appearance of most extant species during the Quaternary may suggest that rapid climate cooling during the Plio-Pleistocene triggered diversification of cryophilic ostracods. These cryophilic ostracods constitute the modern Antarctic fauna showing high endemism and only few of them are found from the South American continental shelf and the Magellan Strait. The scarce knowledge of the modern Antarctic ostracods and in particular of their relationship with environmental parameters and physico-chemical characteristics of the water masses actually represents a strong obstacle in particular for the definition and analysis of environmental, climatic and oceanographic changes that have affected older periods. For this purpose, for a possible interpretation of the environmental and climatic changes that affected the Ross Sea in particular during the Plio-Pleistocene, the ostracod associations found in ninety surface sediment samples taken during different oceanographic research cruises were analyzed. The data obtained were connected with the numerous physico-chemical data of the water masses taken since the 90's by the oceanographic research cruises organized by the National Research Program in Antarctica (PNRA) in the Western Ross Sea area. In this way we obtained the distribution (presence absence) of the ostracod modern species in the Westerns Ross Sea such as induced by the different oceanographic settings and environmental conditions. These results have been compared with high quanti/qualitative ostracod associations values recorded in three units recovered in the IODP_374 Site U1523 that show an equivalent fauna to the modern sediments. So we believe which could be possible, through the comparison between modern and past ostracod associations, to provide a possible reconstruction of the complex environmental, climatic and oceanographic changes that have affected the Ross Sea during the last 3.0 Ma.

Ostracods fauna from IODP_374 Site U1523: a comparison with modern ones from the Western Ross Sea Area.

Geophysical/geological data integration contribute to constrain the reconstruction of the deglaciation patterns imprinted on the seafloor (Anderson et al., 2018; Bart et al., 2020; Prothro et al., 2020). The Northern Drygalski Basin (Ross Sea western coast) shows a subglacial landscape tectonically and ice drainage controlled. Seismo-stratigraphic architectures, related to the fluctuations of the East Antarctic Ice Sheet, allow to interpret the glacial-deglacial sediment depocenters and the Grounding Zone Wedges in this area. Sedimentological, micropaleontological and geochemical analysis on two GLEVORS (Glacial Evolution in the north-western Ross Sea, offshore North Victoria Land – Antarctica)  Project Cores GC01 and GC02, result basic for a reliable geomorphic reconstruction in a context where ice landforms and bedforms were shaped and where grounding lines and melt-water processes took place. The sediment cores, considered as important time-snapshotscan provide a chronostratigraphic control on the past marine-based ice-sheet vulnerability and retreat and on the records of the ice streams draining catchments, the post-LGM biological, cryosphere/atmosphere and oceanographic processes (Kim et al., 2018; Post et al., 2020; Smith et al. 2020).

Ross Sea snapshots from the past – GLEVORS Project indirect and direct records

Over the last few decades, different researches have led to important advances in our understanding of late Quaternary Antarctic Ice Sheet (AIS) behavior. However, the continental shelf margin remains still poorly investigated. To address this issue, we aim to increase the knowledge of the last glacial/deglacial dynamics in the Central Basin (slope-basin system) using a multidisciplinary approach, including sedimentological, micropaleontological, and tephrochronological analyses performed on marine sediments cores. Based on grain size clustering and the sortable silt data, together with diatom, silicoflagellate and foraminifera assemblages, we recognize three stratigraphic units. Unit 1 (24-17 ka BP) documents the influence of the ice shelf calving zone vicinity, Unit 2 (17-10.2 ka BP) represents the progressive retreat of the ice shelf due to Circumpolar Deep Water inflow, and Unit 3 (10.2 ka BP-present) corresponds to the establishment of seasonal sea ice with a strengthening of bottom currents. The dominant and persistent process is the sedimentation controlled by contour currents, which tend to modulate intensity in time and space. The study also allowed us to document the presence of a primary volcanic ash layer dated 22 ka BP correlated with the explosive activity of Mount Rittmann. This study has been conducted in the framework of the STREAM Project (Late Quaternary evolution of the ocean-ice sheet interactions: the record from the Ross Sea continental margin, Antarctica) funded by the Italian Ministry of Foreign Affairs and International Cooperation and the National Research Foundation of Korea.

Environmental and Oceanographic Conditions at the Continental Margin of the Central Basin, Northwestern Ross Sea (Antarctica) since Last Glacial Maximum

In this paper we analyze how oceanic circulation affects sediment deposition along a sector of the Ross Sea continental margin, between the Iselin Bank and the Hillary Canyon, and how these processes evolved since the Late Miocene. The Hillary Canyon is one of the few places around the Antarctic continental margin where the dense waters produced onto the continental shelf, mainly through brine rejection related to sea ice production, flow down the continental slope and reach the deep oceanic bottom layer (Gales et al., 2021). At the same time the Hillary Canyon represents a pathway for relatively warm waters, normally flowing along the continental slope within the Antarctic Slope Current, to reach the continental shelf. The intrusion of warm waters onto the continental shelf produces basal melting of the ice shelves, reduces their buttressing effect and triggers instabilities of the ice sheet that represents one of the main uncertainties in future sea level projections. For this study we use seismic, morpho-bathymetric and oceanographic data acquired in 2017 by the R/V OGS Explora. Age models from two drilling sites (U1523 and U1524) of the Integrated Ocean Discovery Program (IODP) Expedition 374 (McKay et al., 2019). Oceanographic data, together with a regional oceanographic model, are used to support our reconstruction by showing the present-day oceanographic influence on sediment deposition. Regional correlation of the main seismic unconformities allows us to identify eight seismic sequences. Seismic profiles and multibeam bathymetry show a strong influence of bottom current activity on sediment deposition since the Early Miocene and a reduction in their intensity during the mid-Pliocene Warm Period. Oceanographic data and modelling provide evidence that the bottom currents are related to the dense waters produced on the Ross Sea continental shelf and flowing out through the Hillary Canyon. The presence of extensive mass transport deposits and detachment scarps indicate that also mass wasting participates in sediment transport. Through this integrated approach we regard the area between the Iselin Bank and the Hillary Canyon as a Contourite Depositional System (ODYSSEA CDS) that offers a record of oceanographic and sedimentary conditions in a unique setting. The hypotheses presented in this work are intended to serve as a framework for future reconstructions based on detailed integration of lithological, paleontological, geochemical and petrophysical data.

Bottom current control on sediment deposition between the Iselin Bank and the Hillary Canyon (Antarctica) since the late Miocene: an integrated seismic-oceanographic approach

The Cenomanian–Turonian Oceanic Anoxic Event 2 (OAE-2) represents a major perturbation of the global carbon cycle associated with the extensive deposition of organic carbon-rich deposits (black shales) in the oceanic basins, worldwide. Numerous studies have suggested that enhanced volcanic activity at mid-oceanic ridges and large igneous province (LIP) eruptions may have acted as potential trigger mechanisms of this event. These phenomena caused excess CO2 degassing and a consequent dramatic temperature increase in the oceans and atmosphere. Although the geochemical, sedimentological and paleontological changes throughout the OAE-2 are well-known in basinal deposits, the shallow- water carbonate counterpart is still poorly investigated. The OAE-2, however, strongly impacted both deep and shallow-water ecosystems. The limited information on the expression of the OAE-2 in carbonate platforms represents a critical gap in our understanding of ecological responses to abrupt paleoenvironmental/paleoclimatic changes in the mid-Cretaceous, given that shallow shelves accounted for a significant part of the global ocean biological productivity and carbonate production. In this work we present new high-resolution geochemical and sedimentological data across the OAE-2, based on bivalve shells and bulk rock material from a well exposed upper Cenomanian – lower Turonian section, in the carbonate successions of the Friuli Carbonate Platform (FCP) cropping out in the Karst area, near the city of Gorizia. Biostratigraphy and carbon-isotope stratigraphy were used to establish a precise stratigraphic dating and for high resolution correlations with coeval shallow-water and reference open-ocean sections. Sr-isotope data, Hg concentration and other geochemical proxies have been used to highlight possible evidence of volcanic activity and changes in weathering and to evaluate paleoenvironmental conditions on the FCP during the OAE-2. Results highlight that, during the event, the FCP underwent major biotic and geochemical changes that these fluctuations can be compared to those occurring in coeval carbonate platforms of the Tethys during the OAE-2. 

First evidence of the Oceanic Anoxic Event -2 in the shallow-water carbonates of the Friuli Carbonate Platform: geochemical and sedimentological data

Here we present an integrated calcareous plankton and geochemical record from the Tethyan Terche and Madeago sections (NE Italy), deposited in a middle-lower bathyal setting. These sections record a negative Carbon Isotope Excursion corresponding to Eocene Thermal Maximum 2 (ETM2, ~54 Ma). Our planktic foraminiferal and calcareous nannofossil records show significant, though temporary, changes in both sections across this event. No significant dissolution occurred that could have affected the assemblages. Warm index surface-dweller Acarinina increased while of the deeper-dweller chiloguembelinids and subbotinids declined in abundance. A test reduction of up to 50% compared to pre-event sizes for both surface- and deeper dwellers planktic foraminiferal is particularly marked at Madeago. While symbiont bleaching can reduce growth, both symbiont and asymbiotic planktic foraminifers were affected at the same scale in our sections at the ETM2, and hence not a likely cause of the size reduction. The enhanced surface remineralization of organic matter due to warming may have reduced the food input at the thermocline, thus contracting the deeper ecological niches. The calcareous nannofossil assemblages provide evidence of increased surface water eutrophy during the ETM2. The increase in small placoliths Toweius and Ericsonia may indicate that dwarfism also affected nannofossil assemblages, though to a lesser degree. We thus hypothesize that the ETM2 perturbation impacted the entire upper water column possibly changing stratification and niche spaces. To establish whether the observed dwarfism was related to environmental stressors which are not commonly linked to the ETM2 climatic perturbation, we quantified the Hg (ppb), TOC (wt%) and Hg/TOC (ppb/wt%) geochemical proxies through our sections. Results show an increase in Hg in both sections at the base of the ETM2. We attribute this increase to coeval submarine igneous events that might have introduced biolimiting metals, which changed plankton productivity and possibly affected test size. However, we cannot exclude reworking of the metal because we record an increase in sedimentation rate and nannofossil rework at the ETM2. We suggest that the plankton dwarfism, here documented for the first time across the ETM2, could be the result of the coupled effect of increase in biolimiting metals and changes in the physicochemical structure of the upper water column.

Evidence of Tethyan calcareous plankton dwarfism across the ETM2

The increasing effects of anthropogenic carbon dioxide (CO2) emissions on climate change pressure the scientific community to provide accurate predictions of the future consequences of these variations. The oceans and marine organisms are strongly affected by climate and CO2atm, but they also produce pivotal feedback on climate. Thus, studying the marine carbon cycle is crucial to understand the drivers regulating the past, present, and future evolution of climate. Among marine biota, coccolithophores, a group of calcifying marine phytoplankton, play a key role in the ocean carbon cycle at various timescales providing pivotal information on the oceans’ capacity towards CO2 buffering. They are also particularly sensitive to environmental variations making them excellent climate proxies. In this study, experimental cultures of the species Helicosphaera carteri cultivated in turbidostats under CO2-controlled conditions were compared to the fossil coccolithophore assemblage at the International Ocean Discovery Program (IODP) Site U1501 (South China Sea). The combination of paleontological and culture-derived data can improve predictions of the state of biodiversity in the future under varying climate change scenarios. Helicosphaera carteri cultures were grown under 425 ppm CO2, a level consistent with the best-case scenario predicted by the Intergovernmental Panel on Climate Change (IPCC), the Representative Concentration Pathway (RCP) 2.6 (IPCC, 2014). The selected time interval for studying the fossil coccolithophore assemblage belongs to Marine Isotope Stage 5 (130-70 kyr), a geological interval considered as an analogue for modern warming. Through the study of the fossil record, we identified significant changes to the coccolithophore community during the warmest substages of MIS5, i.e. MIS5e. Site U1501 recorded a significant peak in coccolithophore absolute abundance related to increased productivity during MIS5e, and consequently, the highest coccolith-derived carbonate contribution to bulk calcium carbonate of the last 300 kyr. Data from cultures indicated the resilience of H. carteri towards high CO2 through the maintenance of well-preserved coccoliths. However, the growth rate, despite being constant, was slightly lower than the control culture, indicating a decreased contribution to calcite production/export under high CO2 levels. The results suggest coccolithophores may play an important, but shifting role in carbon sequestration in a warmer world. Further, this comparison contributes to improving our understanding of the potential effects of anthropogenic climate change on oceanic primary producers like coccolithophores.

Living and Fossil Coccolithophores: a tool to understand past and future climate changes

In the geological record, calcareous nannofossils are intensively studied, because they are important palaeoecological tracers. Today, as in the past, coccolithophores are really sensitive to environmental changes. So, their growth and calcification rates could be influenced by different factors such as temperature, carbonate chemistry and/or nutrient content (i.e, Erba et al., 2010; Wulff et al., 2020). The variations in nannoplankton abundance, shape and/or coccolith size could give information of past ocean-atmosphere conditions including the amount of CO2 exchange between the ocean and the atmosphere. For this research the time interval selected is the Early Aptian to Late Albian (ca. 121-100 Ma) which was characterized by important climatic and paleoenvironmental changes including a cooling event in the early late Aptian, and two “Ocean Anoxic Events”, namely the OAE 1a and OAE 1b. Particularly, the focus is on the Aptian/Albian boundary and the Oceanic Anoxic Event 1b. The whole study interval was selected because morphometrics analyses on Biscutum constans from the Western Tethys documented relatively small sized specimens in the early late Aptian (Bottini & Faucher, 2020), followed by average size increase after OAE 1b. Here, we conduct new morphometries on species with particular affinities in temperature or fertility such as Watznaueria barnesiae (oligotrophic species), Rhagodiscus asper (warm water species), Zeugrhabdotus erectus and Biscutum constans (mesotrophic species). The aim of the study is to a) see if there are any dimensional variations in different species other than B. constans; b) investigate if the species responded to paleotemperature and paleofertility changes with size variations and c) detect if the coccolith size justifies a decrease in nannoplankton calcification and how this possibly influenced the total carbon budget. Thus, in order to reach these objectives, we studied samples from ODP Site 1049 (Proto-North Atlantic) and the Piobbico core (Umbria-Marche Basin, central Italian Apennines) and we have correlated new morphometrics data with the nannofossil abundance, temperature and fertility index.

Mg/Ca Paleotempertures changes at the demise of plankic foraminiferal genus Morozovella across the Early Eocene Climatic Optimum (Site 1263, Southern Atlantic Ocean)

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