From soil to sea: sources and transport of organic carbon traced by tetraether lipids in the monsoonal Godavari River, India

Monsoonal rivers play an important role in the land-to-sea transport of soil-derived organic carbon (OC). However, spatial and temporal variation in the concentration, composition, and fate of this OC in these rivers remains poorly understood. We investigate soil-to-sea transport of soil OC by the Godavari River in India using glycerol dialkyl glycerol tetraether (GDGT) lipids in soils, river suspended particulate matter (SPM), and riverbed sediments, as well as in a marine sediment core from the Bay of Bengal. The abundance and composition of GDGTs in SPM and sediments in the Godavari River differs between the dry and wet season. In the dry season, SPM and riverbed sediments from the whole basin contain more 6-methyl branched GDGTs (brGDGTs) than the soils. In the upper basin, where mobilisation and transport of soils is limited due to deficient rainfall and damming, contributions of 6-methyl brGDGTs in SPM and riverbed sediments are relatively high year-round, suggesting that they have an aquatic source. Aquatic brGDGT production coincides with elevated values of the isoprenoid GDGT-0  crenarchaeol ratio in SPM and riverbed sediments from the upper basin, indicating low-oxygen conditions. In the wet season, brGDGT distributions in SPM from the lower basin closely resemble those in soils, mostly from the north and east tributaries, corresponding to precipitation patterns. The brGDGT composition in SPM and sediments from the delta suggests that soil OC is only effectively transported to the Bay of Bengal in the wet season, when the river plume extends beyond the river mouth. The sediment geochemistry indicates that also the mineral particles exported by the Godavari River primarily originate from the lower basin, similar to the brGDGTs, suggesting that they are transported together. However, river depth profiles in the downstream Godavari reveal no hydrodynamic sorting effect on brGDGTs in either season, indicating that brGDGTs are not closely associated with mineral particles. The similarity of brGDGT distributions in bulk and fine-grained sediments (≤ 63 µm) further confirms the absence of selective transport mechanisms. Nevertheless, the composition of brGDGTs in a Holocene, marine sediment core near the river mouth appears substantially different from that in the modern Godavari basin, suggesting that terrestrial-derived brGDGTs are rapidly lost upon discharge into the Bay of Bengal and/or overprinted by marine in situ production. The large change in brGDGT distributions at the river–sea transition implies that this zone is key in the transfer of soil OC, as well as that of the environmental signal carried by brGDGTs from the river basin

An expanded database of Southern Hemisphere surface sediment dinoflagellate cyst assemblages and their oceanographic affinities

Dinoflagellate cyst assemblages present a valuable proxy to infer paleoceanographic conditions, yet factors influencing geographic distributions of species remain largely unknown, especially in the Southern Ocean. Strong lateral transport, sea-ice dynamics, and a sparse and uneven geographic distribution of surface sediment samples have limited the use of dinocyst assemblages as a quantitative proxy for paleo-environmental conditions such as sea surface temperature (SST), nutrient concentrations, salinity, and sea ice (presence). In this study we present a new set of surface sediment samples (nCombining double low line66) from around Antarctica, doubling the number of Antarctic-proximal samples to 100 (dataset wsi_100) and increasing the total number of Southern Hemisphere samples to 655 (dataset sh_655). Additionally, we use modelled ocean conditions and apply Lagrangian techniques to all Southern Hemisphere sample stations to quantify and evaluate the influence of lateral transport on the sinking trajectory of microplankton and, with that, to the inferred ocean conditions. k-means cluster analysis on the wsi_100 dataset demonstrates the strong affinity of Selenopemphix antarctica with sea-ice presence and of Islandinium spp. with low-salinity conditions. For the entire Southern Hemisphere, the k-means cluster analysis identifies nine clusters with a characteristic assemblage. In most clusters a single dinocyst species dominates the assemblage. These clusters correspond to well-defined oceanic conditions in specific Southern Ocean zones or along the ocean fronts. We find that, when lateral transport is predominantly zonal, the environmental parameters inferred from the sea floor assemblages mostly correspond to those of the overlying ocean surface. In this case, the transport factor can thus be neglected and will not represent a bias in the reconstructions. Yet, for some individual sites, e.g. deep-water sites or sites under strong-current regimes, lateral transport can play a large role. The results of our study further constrain environmental conditions represented by dinocyst assemblages and the location of Southern Ocean frontal systems

Southern Ocean, Antarctic ice and climate interactions during Neogene cooling

The partial pressure of atmospheric carbon dioxide (pCO2) has increased from 280 to 420 ppm (parts per million) since the industrial revolution due to anthropogenic emissions. As a result, the Earth's atmosphere, ocean, and cryosphere are undergoing changes due to increased radiative forcing, leading to a warming planet, loss of ice sheets and sea ice, sea level rise, ocean heat redistribution, and fluctuations in ocean circulation, as predicted in scenarios up to the year 2100. Nevertheless, future changes in these components remain highly uncertain—better understanding of these processes is crucial for human communities. To better comprehend the interactions between the Southern Ocean, the Antarctic ice sheet (AIS), pCO2, and climate in the future, I have studied sea surface temperatures, ocean front migrations and deep-sea temperatures during climate transitions in the geological past, specifically the Neogene (2.58–23.04 million years ago, Ma), which experienced pCO2 levels as high as, and sometimes higher than, the current levels. A distribution model of modern dinocysts was developed based on newly collected surface sediment samples near Antarctica to reconstruct the positions of oceanic fronts in the Southern Ocean in the past (Chapter 2). Subsequently, I demonstrated a significant long-term cooling of ocean surface and northward migrations of ocean fronts during the Neogene near Tasmania. The substantial cooling at mid-latitudes is not caused by solar radiation combined with polar amplification but is attributed to ocean frontal migrations and the northward expansion of the polar sea. Additionally, I identified a substantial deep-ocean cooling, which nearly completely explains the increase in benthic δ18O, leaving little room for an ice volume effect. The relatively stable ice volume during climate cooling seems counterintuitive given the northward migrations of ocean fronts and other geological evidences suggesting the AIS advancing. Hence, I proposed a hypothesis that the AIS gradually decreased in height while expanding seaward, maintaining a relatively stable volume during the mid-Miocene climatic transition (~14.5 Ma) and verified using ice sheet modelling. The migrations of the subtropical front near Tasmania are closely linked to a high-resolution pCO2 dataset for the Pliocene (2.58–5.3 Ma), which shows a delay of ~10,000 years in pCO2 compared to δ18O during the M2 glaciation and, at least, an elevated pCO2 during glaciation. I proposed that carbon emissions from the deep sea, driven by frontal migrations, was the dominant process in regulating pCO2, rather than physical diffusion or the biological carbon pump. This leads to an higher pCO2 during cold phases because less CO2 is stored in the ocean than during deglaciation. In another crucial geographical area, the Agulhas Plateau, I discovered that migrations of the subtropical front, regulating the inflow of warm water into the Atlantic Ocean, play a significant role in the variability of the Atlantic Meridional Overturning Circulation in the Pliocene. I concluded that there was a connection between frontal migrations and the locality of North Atlantic Deep Water formation, significantly altering heat transport to high northern latitudes and providing insights into understanding future AMOC changes

Glycerol dialkyl glycerol tetraethers in a Holocene sediment core (NGHP-01-16A) in front of the Godavari River in the Bay of Bengal (India)

Isoprenoid and branched Glycerol Dialkyl Glycerol Tetraethers (GDGTs) have been analysed in a marine sediment core (NGHP-01-16A) in front of the Godavari River in the Bay of Bengal (16.59331°N, 82.68345°E, 1268m m water depth). The core covers the Early to Late Holocene (~10000 years). The age model and %TOC are from Ponton et al. (2012) and Usman et al. (2018), respectively. TEX86 ratios and Sea Surface Temperatures were estimated following Kim et al. (2010)

Origin and preservation of archaeal intact polar tetraether lipids in deeply buried sediments from the South China Sea

Intact polar lipids-glycerol dibiphytanyl glycerol tetraethers (IPL-GDGTs) are assumed to be degraded to core lipids (CL) upon cell death, which thus can serve as markers for live archaea in marine deep biosphere. However, the degradation models of sedimentary IPL-GDGTs suggested that they are mainly fossil compounds and can be preserved over geological timescales. Here we investigated the CL- and IPL-GDGTs from deeply buried sediments (0.1-485 mbsf, similar to 7.3 Ma) in the South China Sea (SCS). The depth profiles of IPL-GDGT concentrations paralleled those of CL-GDGTs. The sea surface temperatures (SST) derived from CL- and IPL-TEX 86 ranged 23.1-28.8 degrees C and 22-33.3 degrees C, respectively. They are close to the SST in the SCS, suggesting that CL- and IPL-GDGTs mostly originate from pelagic archaea. The composition and distribution of the IPL-GDGTs differed among the polar headgroups. Hexose-phosphohexose (HPH)-GDGTs were dominated by GDGT-0, monohexose (1G)-GDGTs were mainly composed of GDGT-0 and crenarchaeol, and dihexose (2G)-GDGTs consist primarily of GDGT-2 and crenarchaeol isomer. The compositions of HPH-, 1G- and 2G-GDGTs are similar to those in previously studied water column samples, supporting that sedimentary IPL-GDGTs predominantly derive from the water column. HPH-GDGT abundances fast declined to be undetectable at similar to 31 mbsf (similar to 0.6 Ma) while 1G- and 2G-GDGTs dominated the deeply buried sediments, indicating that GDGTs with glycosidic headgroups are better preserved versus GDGTs with phosphorous headgroups over geological timescales. Our results demonstrate that IPL-GDGTs, especially glycosidic GDGTs, are not suitable as biomarkers for live archaea in the deep biosphere

Late Eocene–early Miocene evolution of the southern Australian subtropical front: a marine palynological approach

Improvements in our capability to reconstruct ancient surface-ocean conditions based on organic-walled dinoflagellate cyst (dinocyst) assemblages from the Southern Ocean provide an opportunity to better establish past position, strength and oceanography of the subtropical front (STF). Here, we aim to reconstruct the late Eocene to early Miocene (37-20ĝ€¯Ma) depositional and palaeoceanographic history of the STF in the context of the evolving Tasmanian Gateway as well as the potential influence of Antarctic circumpolar flow and intense waxing and waning of ice. We approach this by combining information from seismic lines (revisiting existing data and generating new marine palynological data from Ocean Drilling Program (ODP) Hole 1168A) in the western Tasmanian continental slope. We apply improved taxonomic insights and palaeoecological models to reconstruct the sea surface palaeoenvironmental evolution. Late Eocene-early Oligocene (37-30.5ĝ€¯Ma) assemblages show a progressive transition from dominant terrestrial palynomorphs and inner-neritic dinocyst taxa as well as cysts produced by heterotrophic dinoflagellates to predominantly outer-neritic/oceanic autotrophic taxa. This transition reflects the progressive deepening of the western Tasmanian continental margin, an interpretation supported by our new seismic investigations. The dominance of autotrophic species like Spiniferites spp. and Operculodinium spp. reflects relatively oligotrophic conditions, like those of regions north of the modern-day STF. The increased abundance in the earliest Miocene of Nematosphaeropsis labyrinthus, typical for modern subantarctic zone (frontal) conditions, indicates a cooling and/or closer proximity of the STF to the site . The absence of major shifts in dinocyst assemblages contrasts with other records in the region and suggests that small changes in surface oceanographic conditions occurred during the Oligocene. Despite the relatively southerly (63-55°S) location of Site 1168, the rather stable oceanographic conditions reflect the continued influence of the proto-Leeuwin Current along the southern Australian coast as Australia continued to drift northward. The relatively "warm"dinocyst assemblages at ODP Site 1168, compared with the cold assemblages at Antarctic Integrated Ocean Drilling Program (IODP) Site U1356, testify to the establishment of a pronounced latitudinal temperature gradient in the Oligocene Southern Ocean

From soil to sea: sources and transport of organic carbon traced by tetraether lipids in the monsoonal Godavari River, India

Monsoonal rivers play an important role in the land-to-sea transport of soil-derived organic carbon (OC). However, spatial and temporal variation in the concentration, composition, and fate of this OC in these rivers remains poorly understood. We investigate soil-to-sea transport of soil OC by the Godavari River in India using glycerol dialkyl glycerol tetraether (GDGT) lipids in soils, river suspended particulate matter (SPM), and riverbed sediments, as well as in a marine sediment core from the Bay of Bengal. The abundance and composition of GDGTs in SPM and sediments in the Godavari River differs between the dry and wet season. In the dry season, SPM and riverbed sediments from the whole basin contain more 6-methyl branched GDGTs (brGDGTs) than the soils. In the upper basin, where mobilisation and transport of soils is limited due to deficient rainfall and damming, contributions of 6-methyl brGDGTs in SPM and riverbed sediments are relatively high year-round, suggesting that they have an aquatic source. Aquatic brGDGT production coincides with elevated values of the isoprenoid GDGT-0 / crenarchaeol ratio in SPM and riverbed sediments from the upper basin, indicating low-oxygen conditions. In the wet season, brGDGT distributions in SPM from the lower basin closely resemble those in soils, mostly from the north and east tributaries, corresponding to precipitation patterns. The brGDGT composition in SPM and sediments from the delta suggests that soil OC is only effectively transported to the Bay of Bengal in the wet season, when the river plume extends beyond the river mouth. The sediment geochemistry indicates that also the mineral particles exported by the Godavari River primarily originate from the lower basin, similar to the brGDGTs, suggesting that they are transported together. However, river depth profiles in the downstream Godavari reveal no hydrodynamic sorting effect on brGDGTs in either season, indicating that brGDGTs are not closely associated with mineral particles. The similarity of brGDGT distributions in bulk and fine-grained sediments (<= 63 mu m) further confirms the absence of selective transport mechanisms. Nevertheless, the composition of brGDGTs in a Holocene, marine sediment core near the river mouth appears substantially different from that in the modern Godavari basin, suggesting that terrestrial-derived brGDGTs are rapidly lost upon discharge into the Bay of Bengal and/or overprinted by marine in situ production. The large change in brGDGT distributions at the river-sea transition implies that this zone is key in the transfer of soil OC, as well as that of the environmental signal carried by brGDGTs from the river basin.ISSN:1726-4170ISSN:1726-417

From soil to sea: sources and transport of organic carbon traced by tetraether lipids in the monsoonal Godavari River, India

Monsoonal rivers play an important role in the land-to-sea transport of soil-derived organic carbon (OC). However, spatial and temporal variation in the concentration, composition, and fate of this OC in these rivers remains poorly understood. We investigate soil-to-sea transport of soil OC by the Godavari River in India using glycerol dialkyl glycerol tetraether (GDGT) lipids in soils, river suspended particulate matter (SPM), and riverbed sediments, as well as in a marine sediment core from the Bay of Bengal. The abundance and composition of GDGTs in SPM and sediments in the Godavari River differs between the dry and wet season. In the dry season, SPM and riverbed sediments from the whole basin contain more 6-methyl branched GDGTs (brGDGTs) than the soils. In the upper basin, where mobilisation and transport of soils is limited due to deficient rainfall and damming, contributions of 6-methyl brGDGTs in SPM and riverbed sediments are relatively high year-round, suggesting that they have an aquatic source. Aquatic brGDGT production coincides with elevated values of the isoprenoid GDGT-0  crenarchaeol ratio in SPM and riverbed sediments from the upper basin, indicating low-oxygen conditions. In the wet season, brGDGT distributions in SPM from the lower basin closely resemble those in soils, mostly from the north and east tributaries, corresponding to precipitation patterns. The brGDGT composition in SPM and sediments from the delta suggests that soil OC is only effectively transported to the Bay of Bengal in the wet season, when the river plume extends beyond the river mouth. The sediment geochemistry indicates that also the mineral particles exported by the Godavari River primarily originate from the lower basin, similar to the brGDGTs, suggesting that they are transported together. However, river depth profiles in the downstream Godavari reveal no hydrodynamic sorting effect on brGDGTs in either season, indicating that brGDGTs are not closely associated with mineral particles. The similarity of brGDGT distributions in bulk and fine-grained sediments (≤ 63 µm) further confirms the absence of selective transport mechanisms. Nevertheless, the composition of brGDGTs in a Holocene, marine sediment core near the river mouth appears substantially different from that in the modern Godavari basin, suggesting that terrestrial-derived brGDGTs are rapidly lost upon discharge into the Bay of Bengal and/or overprinted by marine in situ production. The large change in brGDGT distributions at the river–sea transition implies that this zone is key in the transfer of soil OC, as well as that of the environmental signal carried by brGDGTs from the river basin