Deep-sea Sediment Cores Research Articles

Chemical and diatom compositions of dating deep-sea sediment core of the Kuril-Kamchatka Trench, northwestern Pacific

For the first time the grain size, contents of major (Al, Ti, Fe, Mn, Ca, Mg, Na, and K) and trace elements (Mo, Ni, and Cu), total organic carbon, and stable carbon isotope were studied in the sediment cores collected at a water depth of approximately 9500 m from the Kuril-Kamchatka Trench. This study determined the factors of sedimentary environment. The quantitative content and species composition of diatoms were also studied. We have estimated the sedimentation rate based on the excess 210Pb data. According to age models, the sedimentation rate was ∼1.64 mm/year. Paleoconstructions were formed on comparing the distribution of chemical and biological parameters in deep-sea basin sediments. The studied sediments were characterized by the high content of the bioproductivity indicators and chemical elements (Fe, Ca, and Cu) corresponding to a period of increased supply of nutrients that stimulated the production of diatoms. Based on the study of chemical indicators and characteristics of nutrient utilization by phytoplankton, a period of outbreak of diatom productivity due to the introduction of volcanic ash into the sediment in the period 1860–1883 was marked. New data will complement existing information and helpful for further understanding of sedimentary geochemical characteristics in this hadal area, and allow us to relate abiotic conditions to productivity.

High-precision U-Pb geochronology for the Miocene Climate Optimum and a novel approach for calibrating age models in deep-sea sediment cores

Abstract Scientific ocean drilling cores recovered years ago (legacy cores), especially as recovered by rotary drilling, commonly show incomplete recovery and core disturbance. We present a novel method to date such cores by presenting the first high-precision U-Pb zircon ages targeting the duration of the Miocene Climate Optimum (MCO; ca. 17–14 Ma) from volcanic ashes at Ocean Drilling Program Site 1000 (on the Nicaragua Rise in the Caribbean Sea). We place these ages within a newly developed framework to address incomplete core recovery and use them to calibrate a high-resolution bulk carbonate δ13C and δ18O record. Our Site 1000 ages show that volcanism of the Columbia River Basalt Group (CRBG) large igneous province was coincident with the interval of greatest sustained MCO warmth at this site. However, if the CRBG were the primary driver of the MCO, our chronology may allow for outgassing preceding volcanism as a major source of CO2. We thus document a promising new way to obtain highly resolved, accurate, and precise numerical age models for legacy deep-sea sediment cores that does not depend on correlation to other records.

East Asian Winter Monsoon and enhanced stratification in the western Pacific promoted giant marine diatom (Ethmodiscus rex) blooms during the last deglaciation

Late Pleistocene thick laminated mats of Ethmodiscus rex (E. rex) have been found on the seafloor of the western tropical Pacific. Exploring the causes of E. rex mats formation can improve our understanding of the role of diatom mats in the global carbon-silicon cycle. Nutrients, such as silicate, are a major factor contributing to the growth of diatoms. However, the origin of the nutrients necessary for E. rex to bloom remains unclear. Here we analysed the 14C ages and silicon isotope compositions (δ30Si) of E. rex mats from deep-sea sediment cores collected in the western Pacific to reveal the nutrient source of the diatom bloom. The results indicate that the age of the cores ranges from 11 ka to 19 ka. This is the first discovery of a last deglacial thick layer of E. rex mats in the western Pacific Hadal zone. Furthermore, the silicon isotope results show that, similar to previous studies in the western Pacific, all δ30Si of E. rex frustules are negative. This is in contrast to the North Atlantic upwelling region where all δ30Si are positive. According to the binary mixing model calculations, eolian dust were responsible for supplying 80%–90% of the silica during the last deglaciation, while upwelling waters accounted for only about 10%–20% of the silica. During Heinrich Stadial 1 (HS1) period, the strengthened East Asian Winter Monsoon (EAWM) transported a lot of eolian dust to the western Pacific, providing nutrients for diatom blooms. In addition, the enhanced stratification in the western Pacific since the late HS1 period was a significant factor that contributes to the blooms of E. rex when the supply of eolian dust material decreased.

Far-field effects of the Nile damming on the silica cycle in the Southeastern Mediterranean Sea

Silica plays a key role in the growth of silicifying primary producers (e.g., diatoms) and hence the ocean carbon pump. The Mediterranean Sea's eastern Levantine Basin (ELB) is a low silica (and low N and P) ultra-oligotrophic basin. Before 1965, Nile autumn floods were a major source of dissolved silica (DSi) and other nutrients to primary producers of the ELB continental shelf, also known as the Nilotic cell. The construction of the Aswan High Dam (AHD) in the mid-1960s, blocked these floods, drastically diminishing the autumn-diatom blooms offshore the Nile delta. However, the far-reaching and long-lasting effects of the Nile damming on the Si cycle in the ELB remain unclear. Here, we studied the changes in DSi in the surface water offshore Israel and the distribution of biogenic silica in deep-sea short sediment cores, collected hundreds of kilometers from the Nile outlet, at depths range of 1100–1900 m, offshore the ELB Israeli coast. We show post dam reduction and termination in flood related seasonality of DSi and a concurrent decrease (of up to 79 %) in biogenic silica (BSi) accumulation rates in surficial sediments relative to underlying sediments. These changes reflect the effects of Si (dissolved and particulate) retention by the AHD on diatoms production, export and burial in the ELB. This far-field effect was demonstrated in deep-sea areas subjected to intense lateral transport of resuspended sediments from the shelf via intermediate nepheloid layers and to coastal water intrusions, along the path of the pre-dam, flood plumes. Our core records show that the AHD worsened nutrient-diminished, exceptionally unfavorable conditions for diatoms that persisted in the deep ELB at least during the last four millennia.

Elevated bacterial endospores associated with thermogenic hydrocarbon seeps in deep sea sediments.

Introduction and approach Bacterial endospore distributions in marine sediments are influenced by geological conduits providing routes for subsurface to surface microbial dispersal. To examine this phenomenon in more detail, endospore abundance was determined by quantifying the biomarker 2,6-pyridine dicarboxylic acid (dipicolinic acid or DPA) in 16 deep sea sediment cores from hydrocarbon prospective areas in the NW Atlantic Ocean. DPA is specific to endospore-forming bacteria from the phylum Firmicutes and constitutes a significant percentage of endospore dry weight. DPA is therefore a potential biomarker for sediment dwelling endospores and geological conduits. Piston cores (10), gravity cores (3) and box cores (3) were collected during two expeditions to the Scotian Slope in the NW Atlantic Ocean off the east coast of Canada aboard the CCGS Hudson in 2016 and 2018 (Campbell (2016), Campbell and Normandeau (2018), Campbell and MacDonald. (2016)). Sampling sites were 1970 to 2791 m water depth, with piston cores (n=3) ranging from 344 to 953 cmbsf and gravity cores (n=10) ranging from 43 to 739 cmbsf, box coring captured the top 25 cmbsf. To address the efficacy of DPA biomarker analysis as a tool for hydrocarbon seep location we established a modified Tb3+ chelation method (Lomstein and Jørgensen (2012), Rattray (2021)). Sediment samples were extracted using acid hydrolysis, chelated with Tb3+ and analysed using HPLC fluorescence, measuring at 270 nm emission and 545 nm excitation. DPA concentrations were converted to Endospore numbers were calculated using 2.24 fmol DPA per endospore (Fichtel 2007), a conversion factor routinely used in other studies (Braun 2017, Gittins 2022, Heuer 2020, Lomstein 2012, Rattray 2022, Wörmer 2019, Lomstein and Jørgensen 2012). DPA concentrations were compared with measurements of over 250 different gaseous and liquid hydrocarbon compounds used to assess for the presence of thermogenic hydrocarbons. Results and discussion Samples and locations were assessed as being thermogenic hydrocarbon gas positive (stations 16-41, 18-07) or thermogenic hydrocarbon negative based on the abundance of C1-C5 hydrocarbons in sediments sampled from the same cores. Station 18-14 contained hydrocarbons from biogenic origin. Station 18-06 is the only site with higher endospore abundance but that was determined to be hydrocarbon negative. Deep water Scotian Slope sediment cores show high endospore abundance correlates with thermogenic hydrocarbon seeps (Fig. 1). Cores from locations lacking evidence for thermogenic hydrocarbons generally contained significantly lower endospore abundances, with the notable exception of site 18-06. This potential paleoenvironmental hydrocarbon seep site highlights the utility of a DPA proxy for potentially identifying ancient hydrocarbon seeps and investigating past geological systems. The association of high endospore abundances with thermogenic hydrocarbons and the quantity of gas expulsion points to an interesting new biological tool for understanding hydrocarbon seepage in the deep biosphere, based on DPA assays in marine sediments.

Black Carbon in Deep-Sea Seamount Sediment Cores: Vertical Variation and Non-negligible Char Black Carbon.

Deep-sea sediments (>1000 m) are often considered to be the ultimate sink for black carbon (BC), and the long-term buried BC in these sediments is believed to potentially provide a negative feedback effect on climate warming. The burial flux of BC in marine sediments is predominantly estimated based on soot BC (SBC) in most studies, frequently ignoring the contribution of char BC (CBC). While this methodology may result in an underestimation of the BC burial flux, the precise extent of this underestimation is yet to be determined. This study used the benzene poly(carboxylic acid) (BPCA) method and chemothermal oxidation (CTO) method to analyze CBC and SBC in four deep-sea sediment cores from the Zhongnan seamount in the South China Sea, respectively. The CBC content increased from 0.026 ± 0.010% at the seamount upper part (1432 m) to 0.039 ± 0.012% at the seamount foot (4278 m), constituting approximately 25 to 42% of the SBC content. The content disparity between CBC and SBC diminishes as depth increases. In deep-sea sediments, biogeochemical factors influence the variation of CBC molecules with depth. In the seamount middle-upper part (1432 and 2465 m), highly condensed CBC gradually accumulated along the core downward profile. In the sediment core profile of the seamount middle-lower part (3497 m), benzenetricarboxylic acid and benzenetetracarboxylic acid content decreased while the BC condensation degree rose, i.e., less condensed CBC was preferentially consumed. Afterward, CBC molecules reached a relatively stable state at the seamount foot. This study reveals that CBC possesses the capacity for long-term carbon sequestration in deep-sea sediments, and its content is not negligible.

Amplified surface warming in the south-west Pacific during the mid-Pliocene (3.3–3.0 Ma) and future implications

Abstract. Based on Nationally Determined Contributions concurrent with Shared Socioeconomic Pathways (SSPs) 2-4.5, the IPCC predicts global warming of 2.1–3.5 ∘C (very likely range 10–90th percentile) by 2100 CE. However, global average temperature is a poor indicator of regional warming and global climate models (GCMs) require validation with instrumental or proxy data from geological archives to assess their ability to simulate regional ocean and atmospheric circulation, and thus, to evaluate their performance for regional climate projections. The south-west Pacific is a region that performs poorly when GCMs are evaluated against instrumental observations. The New Zealand Earth System Model (NZESM) was developed from the United Kingdom Earth System Model (UKESM) to better understand south-west Pacific response to global change, by including a nested ocean grid in the south-west Pacific with 80 % greater horizontal resolution than the global-scale host. Here, we reconstruct regional south-west Pacific sea-surface temperatures (SSTs) for the mid-Pliocene warm period (mPWP; 3.3–3.0 Ma), which has been widely considered a past analogue with an equilibrium surface temperature response of +3 ∘C to an atmospheric CO2 concentration of ∼350–400 ppm, in order to assess the warming distribution in the south-west Pacific. This study presents proxy SSTs from seven deep sea sediment cores distributed across the south-west Pacific. Our reconstructed SSTs are derived from molecular biomarkers preserved in the sediment – alkenones (i.e. U37K′ index) and isoprenoid glycerol dialkyl glycerol tetraethers (i.e. TEX86 index) – and are compared with SSTs reconstructed from the Last Interglacial (125 ka), Pliocene Model Intercomparison Project (PlioMIP) outputs and transient climate model projections (NZESM and UKESM) of low- to high-range SSPs for 2090–2099 CE. Mean interglacial equilibrium SSTs during the mPWP for the south-west Pacific sites were on average 4.2 ∘C (1.8–6.1 ∘C likely range) above pre-industrial temperatures and show good agreement with model outputs from NZESM and UKESM under mid-range SSP 2–4.6 conditions. These results highlight that not only is the mPWP an appropriate analogue when considering future temperature change in the centuries to come, but they also demonstrate that the south-west Pacific region will experience warming that exceeds that of the global mean if atmospheric CO2 remains above 350 ppm.

Planktonic foraminiferal assemblages reflect warming during two recent mid-latitude marine heatwaves

Under future climate scenarios, ocean temperatures that are presently extreme and qualify as marine heatwaves (MHW) are forecasted to increase in frequency and intensity, but little is known about the impact of these events on one of the most common paleoproxies, planktonic foraminifera. Planktonic foraminifera are globally ubiquitous, shelled marine protists. Their abundances and geochemistry vary with ocean conditions and fossil specimens are commonly used to reconstruct ancient ocean conditions. Planktonic foraminiferal assemblages are known to vary globally with sea surface temperature, primary productivity, and other hydrographic conditions, but have not been studied in the context of mid-latitude MHWs. For this study, the community composition and abundance of planktonic foraminifera were quantified for 2010-2019 along the Newport Hydrographic Line, a long-term monitoring transect at 44.6°N in the Northern California Current (NCC). Samples were obtained from archived plankton tows spanning 46 to 370 km offshore during annual autumn (August – October) cruises. Two MHWs impacted the region during this timeframe: the first during 2014-2016 and a second, shorter duration MHW in 2019. During the 2014-2016 MHW, warm water subtropical and tropical foraminifera species were more prevalent than the typical polar, subpolar, and transitional species common to this region. Cold water species were abundant again after the first MHW dissipated in late 2016. During the second, shorter-duration MHW in 2019, the assemblage consisted of a warm water assemblage but did not include tropical species. The foraminiferal assemblage variability correlated with changes in temperature and salinity in the upper 100 meters and was not correlated with distance offshore or upwelling. These results suggest that fossil foraminiferal assemblages from deep sea sediment cores may provide insight into the magnitude and frequency of past MHWs.

Metabarcoding analysis of meiobenthic biodiversity along the Gulf of Mexico continental shelf

This study explores how diverse the meiobenthic (meiofauna and other benthic micro-eukaryotes) community is throughout the United States Gulf of Mexico (GOM) continental shelf. In late 2010 and 2011, 51 sediment samples were collected along GOM from Texas through Florida at a range of depths (40m–496m). An additional six deep-sea slope sediment cores were collected in December 2010 near the Deepwater Horizon platform (1370–1385m and 1865m). Metabarcoding of the 18S hypervariable V9 region was conducted to assess biodiversity. Within continental shelf samples, there was greater meiobenthic diversity off the Eastern GOM coast in comparison to both Central and Western GOM coast locations. The Eastern GOM coast has known sediment differences from Western GOM sites. These sediment differences along with influences from the Gulf of Mexico Loop Current may account for observed variations in GOM meiobenthic diversity.

The Presence of Microplastics in Ocean Waters and Deep Marine Sediments: Implications for the Gulf of Guinea

Microplastics are ubiquitous in marine environments as they are found in surface waters, across the water column and in deep-sea sediments. Microplastics could adversely affect marine biota on a cellular level by stretching lipid membranes thereby shortening the lifespan of the organism. The ingestion of microplastics has been reported among larger animals with the risk of neurotoxicity and genotoxicity. In this paper, the likely mechanisms determining distribution and abundance of microplastics in ocean waters and deep-sea sediments have been reviewed. While deep-sea sediment cores reveal high microplastic concentration in ocean trenches and submarine canyons, limited data is available on the abundance of microplastics especially in the Gulf of Guinea. The need for monitoring microplastic concentration and more importantly preventing further transport from land sources is expedient.

Elevated bacterial endospores associated with thermogenic hydrocarbon seeps in deep sea sediments

Endospore distributions in marine sediments are influenced by geological conduits providing routes for subsurface to surface microbial dispersal. To examine this phenomenon in more detail, endospore abundance was determined by quantifying dipicolinic acid (DPA) in 16 deep sea sediment cores from hydrocarbon prospective areas in the NW Atlantic Ocean. DPA concentrations were compared with measurements of over 250 different gaseous and liquid hydrocarbon compounds used to assess for the presence of thermogenic hydrocarbons. This revealed significantly higher levels of endospores at hydrocarbon seep sites than at most hydrocarbon-negative sites. In one exceptional case, a hydrocarbon-negative core in close proximity to adjacent thermogenic seep sites exhibited high endospore abundance, indicating either deposition of endospores emitted from the subsurface via nearby seepage, or alternatively a historical thermogenic hydrocarbon seep where hydrocarbons can no longer be detected by standard geochemistry approaches. This work expands the application of DPA as a proxy for quantifying dormant endospore biogeography of both recent and paleoenvironmental hydrocarbon seep sites.

ASH LAYERS FROM SOUTH ANDAMAN SEA: PROBABLY SOURCED FROM TOBA CALDERA

Deep Sea sediment core PC-1 from the South Andaman Sea (7 ̊19.85' N; 94 ̊ 39.26' E; in East Andaman Basin) below the water depth of 3144 m contain discrete ash layers at various depths. According to morphological study, these ash layers contain glass shards of different varieties i.e. Type-I, Type-II, Type-III, Type-IV and Type-V and it is comparable to glass shards of Toba volcanic reported from other parts of the world. This observation is also supported on the basis of relative biostratigraphic datum observed in the core PC-1. The Layer-A (56 cm thick) at 210 cm bsf is just above the biostratigraphic datum of ca. 0.12 Ma, correspond to Youngest Toba Tuff (YTT), followed by Layer-C belongs to Middle Toba Tuff (MTT) and Layer-D inferred as Oldest Toba Tuff (OTT). This interpretation is further supported by the geochemical data obtained from the EDX analysis, which suggest high silica and alkali contents of rhyolitic composition. Hence, geochemical composition, morphology and biostratigraphic data of these discrete tephra layers show identical characteristics to the products of Toba eruptions, including YTT, MTT and OTT.

Mantle helium in pore fluids of deep-sea sediments around the Azores archipelago

We present noble gas concentrations and helium isotope ratios measured in the pore fluids of two deep-sea sediment cores taken at the Azores Plateau. NeXe concentrations were used to estimate the atmospheric noble gas component within the pore fluids and to then determine the terrigenic helium component. The concentrations of terrigenic 3He indicate the presence of hydrothermal fluids and a mantle He component within the sediment pore space. At both sampling locations, the concentration of terrigenic helium increases with sediment depth. The observed concentration gradients suggest a diffusive transport of He through the sediments. The helium isotope ratio of the source of the terrigenic helium is characterized by a typical mid-ocean ridge basalt (MORB). These results support previous studies suggesting that the pelagic sediments surrounding the Azores host active hydrothermal systems.Our data indicate that the transport of terrigenic helium takes place diffusively over larger areas of the ocean floor, and that the concentration of terrigenic helium in the pelagic sediments increases towards the Mid-Atlantic Ridge and Terceira Rift.The data further suggests that in the Azores region the terrigenic noble gas component is only detectable in the sediment pore fluids and cannot be observed in the overlying bottom water. This highlights the potential of measuring noble gas concentrations and isotope ratios in the sediment column to identify hydrothermal activity in the ocean.

Indian Summer Monsoon variability 140–70 thousand years ago based on multi-proxy records from the Bay of Bengal

Understanding the Indian Summer Monsoon (ISM) behaviour during the late Pleistocene has been largely based on the wind-driven upwelling records from the Arabian Sea. However, it remains unclear the extent to which these records can also be used to infer a concomitant signal of monsoon rainfall, or how the two ISM components, rainfall and wind, are linked on millennial timescales. In order to isolate a primary signal of ISM rainfall, we exploit two deep sea sediment cores from the northern Bay of Bengal (Site U1446) and Andaman Sea (Site U1448), both situated proximal to the South Asian continent, and thus ideally situated for capturing ISM rainfall and fluvial runoff. By comparing our multi-proxy ISM rainfall and runoff records with published ISM wind-driven records from the Arabian Sea, we observe pronounced decoupling of the rainfall and wind components of the ISM across Marine Isotope Stage 5/6 (∼140–70 thousand years ago). We reveal that the relative dominance of barometric dynamics (wind) and the thermodynamic (rainfall) components of the monsoon shifts with changes in background climate state. This finding constitutes an important consideration for the interpretation of past monsoon reconstructions. By comparing our new ISM rainfall records with high latitude climate records, we show that moisture export from low-latitudes, via the monsoon, could have preconditioned the high latitudes for ice sheet growth during glacial inceptions.

Distribution and source of plutonium in sediments from the southern Gulf of Mexico

Here, we report on new data (75 analyses) of plutonium (Pu) isotopes to elucidate activity concentrations, inventories, sources, and their transport from the ocean surface to the seafloor from a collection of six deep-sea sediment cores (depths ranging from 257 to 3739 m) in the southern Gulf of Mexico. Sediment cores collected from the continental shelf and upper slope region of the Gulf of Mexico showed 240Pu/239Pu ratios of 0.15 to 0.26, and 239+240Pu-inventories ranging from 14.7 to 33.0 Bq m−2. Inventories and ratios are consistent with global fallout Pu for this tropical region. In contrast, sediment cores collected from the lower slope region and abyssal plain showed low 240Pu/239Pu ratios of 0.07 to 0.13 and much lower 239+240Pu inventories below 6.8 Bq m−2. This implies that only a small fraction of the expected global fallout Pu has reached the deep-sea sediments. The low 240Pu/239Pu isotope ratios indicate that fallout from the Nevada testing site was an important source of Pu in deep-sea sediments, and that this Pu was likely more efficiently scavenged from the water column than Pu from global fallout. We estimated that up to 44% of the total inventory of 239+240Pu in deep-sea sediments is due to the Nevada source. Low values and a progressive decrease of 240Pu/239Pu ratios and 239+240Pu inventories with increasing water depth have been previously reported for the Gulf of Mexico. Analysis of Pu isotopes in two sediment traps from the upper slope regions shows 240Pu/239Pu ratios comparable to those observed in global fallout. These results indicate that global fallout Pu is currently the main source of Pu in sinking particles in the water column. Therefore, a significant fraction of global fallout Pu must still be present, either in a dissolved phase, or as biologically recycled material in the water column, or scavenged on the shelf and shelf break. Our results bring to light important questions on the application of Pu isotopes to establish sediment chronologies in deep-sea sediments, since global fallout features such as the 1963 maximum are not available.

Comment on "An early Miocene extinction in pelagic sharks".

Sibert and Rubin (Reports, 4 June 2021, p. 1105) report an early Miocene extinction in pelagic sharks based on the loss of shark denticle diversity in two widely separated deep-sea sediment cores. We assert that the pattern observed is not a consequence of extinction but results from shifting species ranges induced by global current reorganization.

An analysis of how deep-sea sediments and ice cores provide a record of Quaternary climate change

Photo: © NASA’s Goddard Space Flight Center/Ludovic BruckerThis article explores the range of methods available to analyse ice and deep-sea sediment cores. It also explores the value of such cores ...

The Last 1200 Years of Rainfall/Runoff Variability along the Central Mexico Pacific Coast Associated with the North American Monsoon

This study presents new evidence for long-term variability in the late Holocene North American Monsoon (NAM), Pacific coast of Mexico. We have carried out a rock magnetic study on two deep-sea sediment cores from the Pacific coast Pescadero Basin. The magnetic intensities estimate total magnetic material and are a proxy for total clastic sediment. Ratios of magnetic intensities estimate the grain size of magnetic material. The rock magnetic data show a decimeter scale, multi-decadal oscillation with fourteen cycles (A-N) over the last 1200 years. These oscillations reflect alternating intervals of stronger/coarser magnetic/clastic flux to the coastal ocean and intervals of weaker/finer magnetic flux. We think these variations are caused by variations in long-term dominance of the NAM; summer (wet) monsoons produce rainy conditions (with runoff) while winter (dry) monsoons produce significant offshore winds, increased upwelling/biological productivity. We can correlate our variability to two other published studies southeast of Pescadero Basin, coastal lake sediments in Laguna de Juanacatlan and a Juxtlahuaca Cave stalagmite. Both of these studies estimate local rainfall. We see evidence of the same pattern of multi-decadal rainfall-runoff variability in these records as we see in Pescadero Basin, which is synchronous to within ±25 years over the last 1200 years. The multi-dacadal pattern of hydrologic variability in all three records varies in cycle duration from ~90-years wet/dry cycles in the Little Ice Age (1400–1850 AD) to ~60-years cycles in the Medieval Climate Optimum (1100–1400 AD). This variability in cycle duration suggests some chaotic nature to the regional NAM climate pattern or some long-term non-linear forcing (PDO?).

Clay-mineral distribution in recent deep-sea sediments around Taiwan: Implications for sediment dispersal processes

Clay-mineralogy study of Taiwanese river-mouth sediments, recent deep-water seafloor sediments around Taiwan, along with sediments collected from the Tainan shelf edge, have been investigated to access the source and transport of detrital fine-grained sediments. We determined the clay mineralogy in both hemipelagites and turbidites in the top 50 cm of the deep-sea sediment cores to infer how sediments are dispersed through river-fed turbidity currents, hypopycnal plumes, and oceanic currents. Our results show that the clay mineral assemblages in both hemipelagites and turbidites of different provinces change gradually between two major end-members: illite+chlorite and smectite. They are predominantly sourced from Taiwan and Luzon, respectively. The relative abundances of clay minerals in turbidites and hemipelagites are quite similar in most of the cores. Therefore, we argue that the adjacent turbidites and hemipelagites of a core share common detrital clay sources. We found that smectite is relatively abundant around Taiwan, indicating that the Kuroshio Current is an important transportation system, which brings smectite from Luzon. Besides, the river-related canyon systems consist dominantly of illite and chlorite, and less smectite, indicating that the smectite brought about by the Kuroshio Current is diluted by river-fed hyperpycnal and hypopycnal flows. This also implies that flood-induced turbidity currents are efficient agents for transporting Taiwan-derived sediments into the neighboring deep-sea basins.

Paleoproductivity shifts since the last 130 ka off Lakshadweep, Southeastern Arabian Sea

Marine sediment deposited on the ocean floor and near coastal areas, the western coastal regions of India provide records of monsoonal shifts and productivity. To understand the paleoproductivity in the northern Indian Ocean, we analyzed carbon, hydrogen, nitrogen, sulfur, total organic carbon (TOC) and calcium carbonate (CaCO3) in a deep-sea sediment core collected using gravity corer (GC-01) off Lakshadweep, Southeastern Arabian Sea. The results were then compared with the previous data generated from the surrounding area sediment cores to understand the productivity variations since the last 130 ka. The CaCO3 content in the sediment core varies from 40.82% to 62.48% (with a mean value of 51.96%) and it is noted that these values were lower during the glacial episodes (Marine Isotope Stages-2 and 4) than the interglacial episodes (MIS-1, 3 and 5). The C/N ratio varied from 0.14 to 34.25, but was less than 9, since ∼74 ka to recent, suggesting a marine origin for the organic carbon. The C/N ratio fluctuated significantly during MIS-5, and relatively the highest C/N ratio was observed at 5e ∼127, 5d ∼110 and 5b ∼85 ka, corresponding to stadials 5b and 5d, (except 5e) indicating terrestrial OC from C3 plants. The low C/N ratios during ∼128, ∼102, ∼76 and ∼32 ka match with the interstadials especially during MIS 5 (5a, 5c and 5e), correspondingly, and are marine OC in the source. This suggests that the MIS-5 stadial was interrupted via land source signifying higher productivity owing to the strong southwest monsoon during these periods. Further, high productivity was also observed during the Last Glacial Maximum (LGM) and Holocene in the Southeastern Arabian Sea since the 130 ka.