Surface Water Productivity Research Articles

Glacial-Interglacial Cycles over the Past 150,000 years of South-Eastern Arabian Sea from δ18O Record

This study aims to reconstruct the surface water productivity and related monsoonal intensity along the southwestern continental margin of India, over the last 150,000 years by utilizing the temporal variations of δ18O and δ13C isotopes in the planktic foraminifers (Globigerinoides ruber) from a sediment core obtained from the continental slope off Kochi, south-eastern India. The chronostratigraphy of the core is established on the basis of high resolution stable isotope record of planktic foraminifera (Globigerinoides ruber) and a few AMS radiocarbon ages. Down-core records of δ18O and δ13C show millennial scale variations of primary productivity and monsoonal intensity during past 150 kyr. Boundaries of the Marine isotope stages (MISs) were delineated in the core based on the visual comparison of the δ18O with Martinson’s (D. M. Martinson, N.G. Pisias, J.D. Hays, J. Imbrie, T.C. Moor Jr, N.J. Shackleton, Age dating and the orbital theory of the ice ages: development of a high resolution 0 to 300000 year chronostratigraphy, Quaternary Research, 1987 27, p 1-29) and the SPECMAP data. From these studies it is inferred that the bottom of the core is about 150 kyrs, demarcating the paleoclimatic/paleoceanographic information covering MIS 1 to 6. Boundaries among MIS 6 and MIS 5, MIS 5 and MIS 4, MIS 4 and MIS 3, MIS 3 and MIS 2 and MIS 2 to MIS 1 (Holocene) have been demarcated at depths 11.47 m, 4.68 m, 4.03 m, 18 m, and 21 m respectively in the core studied. MIS 5 section is characterized by alternating warm and cold periods as observed in Polar Ice cores. The main findings of this work indicates that the core site has recorded higher productivity with increased fluvial input during the warm, humid interglacial periods and interstadials than the stadials during the glacial period. However, the temporal variability from 150 kyr to 130 kyr has been considered as the glacial period with cold and dry climate that corresponding to the MIS 6, MIS 4 within this an episode of extremely low terrigenous input with heavier δ18O values is noticed from 12.91 to 12.67 m. Similarly, a period of heavy δ18O values, with less surface runoff and with low δ13C values between 4.03 to 4.68 m bsf has been interpreted as MIS 4. Episodic fluctuation in SW monsoon induced productivity during MIS 3 has been observed in present study. The de-glacial event in MIS 2 marked in present core characterized by an abrupt increase in productivity during the mid-MIS 2.

Palaeoecological Conditions in the South-Eastern and Western Baltic Sea during the Last Millennium

We present the reconstruction of palaeoenvironmental conditions in the Gdansk, Bornholm, and Arkona Basins of the Baltic Sea over the last millennium. A multiproxy study (including geochemical, XRF, grain size, AMS, and micropalaeontological analyses) of five short sediment cores was performed. The relative age of the sediments was determined based on the Pb distribution along the sediment sequences, as radiocarbon dating has resulted in an excessively old age. The retrieved cores cover two comparable warm periods, the Medieval Climate Anomaly and the Modern Warm Period, for which the increase in surface water productivity was reconstructed. Notably, the production of diatoms was higher during the colder periods (the Dark Ages and Little Ice Age), but this was also the case within the Modern Warm Period. In the Gdansk Basin, the initial salinity increase during the Littorina transgression started after 7.7 cal. a BP. The increased inflow activity was reconstructed during the Medieval Climate Anomaly, even in the Gdansk Basin, despite, in general, very low foraminiferal amounts and diversity. The strongly positive North Atlantic Oscillation Index during this period led to the prevalence of westerly winds over the Baltic region and stronger saltwater intrusions. In the recent sediments, the reconstructed inflow frequency demonstrates a variability against the reduction trend, and a general decline compared to the Medieval Climate Anomaly is seen.

Iodate reduction by marine aerobic bacteria.

Iodate reductase (Idr) gene cluster (idrABP1P2 ) is involved in bacterial iodate (IO3 -) respiration under anaerobic conditions. Putative idr gene clusters are present in both anaerobic and aerobic bacteria; however, the specific physiological roles of idr genes in aerobic bacteria remain unclear. Therefore, in this study, three marine aerobic bacteria with putative idr gene clusters (Roseovarius azorensis, Notoacmeibacter marinus, and Aliiroseovarius sediminilitoris) were grown in the presence of iodate to determine whether they can reduce iodate to iodide (I-). All tested bacteria almost completely reduced 2 mM iodate under static conditions but only reduced 0.1-0.5 mM iodate under shaking conditions. Moreover, the washed cell suspension of R. azorensis reduced iodate only when the cells were pre-grown statically in the presence of iodate. Transcriptional analysis revealed that the expression levels of idrA, idrB, idrP1 , and idrP2 genes were upregulated in R. azorensis when the cells were grown statically in the presence of iodate. Specifically, idrA expression was induced by 0.1 μM iodate and was up to 14-fold higher compared to that of the non-iodate control. These results suggest that marine aerobic bacteria reduce iodate under oxygen-limited conditions, and that this capacity is induced by environmentally relevant levels of iodate in seawater. Our results suggest that marine aerobic bacteria contribute to iodide production in marine surface waters, thereby affecting the global iodine cycling and ozone budget.

A multidecadal study of the Malabar upwelling system influencing Indian Mackerel landings along the coasts of Karnataka and Kerala, south-east Arabian Sea

Indian Mackerel (Rastrelliger kanagurta) is an essential small pelagic fishery resource, contributing to nearly 30 % of India's total marine fish landing. However, this species' life cycle and stock status are poorly understood and data deficient. Ocean mesoscale events are also known to influence their landing, besides the environmental factors like- salinity, chlorophyll, and sea surface temperature, affecting these small pelagic fishes' availability, migration, feeding, and reproductive activity. The Malabar upwelling region (southwest coast of India) is a significant upwelling system where upwelling occurs during the monsoon months. The nutrient-rich water increases the productivity of surface water, leading to plankton abundance. This productivity sustains a fishery for several commercially important fishes, mainly small pelagics such as sardines, mackerels, and anchovies, supporting India's most significant coastal pelagic fishery. For understanding the relationship between the target fish and the oceanographic events, in the present study, Pearson's correlation has been estimated between Indian Mackerel landing, rainfall, Sea water temperature at 0, 10, 20, 30, 45, and 50 m depths, mixed layer depth (MLD), their anomalies and occurrences of potential fishing zone (PFZ) lines along the Malabar upwelling region and corresponding coasts of Karnataka and Kerala. Mackerel landing time series showed a significant autocorrelation in four-quarter lag, correlation with rainfall anomaly in one-quarter lag, with PFZ line and MLD in three-quarter lag, and with SWT@50 (Seawater temperature@50 m depth) and SST anomaly in two-quarter lag both in Karnataka and Kerala. Upwelling events, as indicated by the presence of PFZ lines, were found to significantly impact the landings of Indian mackerel along the Karnataka and Kerala coasts. Polynomial equations were used to model the relationship between mackerel landings and these environmental factors, effectively capturing the influence of these parameters on mackerel catch trends.

Winter entrainment drives the mixed layer supply of manganese in the Southern Ocean

AbstractDespite the subnanomolar dissolved manganese concentrations that can co‐limit Southern Ocean primary production, their physical supply mechanisms during winter, for biological consumption in spring and summer have not yet been explored. During austral winter and spring 2019, two cruises were conducted in the Atlantic sector of the Southern Ocean, to determine the distribution and surface water supply mechanisms of dissolved manganese in the upper water column. The supply mechanisms were used to calculate the total flux of dissolved manganese to productive surface waters and were compared to biological consumption estimates. Mean dissolved manganese concentrations in the upper water column (< 500 m) during winter and spring were comparably low (≤ 0.34 nmol kg−1; p > 0.05), with seasonal mixed layer reservoir sizes averaging 65.21 ± 12.93 and 21.64 ± 19.32 μmol m−2, respectively. Winter entrainment contributed 89.33–99.99% (average 97.26% ± 5.28%) of the total dissolved manganese flux, while diapycnal diffusion contributed 0.52–10.58% (average 4.92% ± 5.14%), was identified as the dominant mechanisms for transporting dissolved manganese into the mixed layer in the subantarctic zone, polar frontal zone, and antarctic zone. Here, the winter physical supply rates were higher than the estimated consumption rates during spring, meeting phytoplankton biological demands. Whereas in the subtropical zone, the supply rates were lower than the consumption rates, indicating the presence of additional supply mechanisms such as coastal upwelling, which may help to meet the biological demands in this region.

Spatiotemporal variation of sterols in sediment as markers of primary production and ocean sewage dumping in the southwestern East Sea (Japan Sea)

The sedimentation of organic carbon in the Ulleung Basin, in the southwestern East Sea (Japan Sea) was investigated using radiocarbon and sterols. The accumulation rates of organic carbon and the contents of brassicasterol and dinosterol were higher on the slope than in the central basin, reflecting the surface water productivity, whereas cholesterol showed similar or higher contents in the central basin. The coprostanol concentration in surface sediments reflected the dispersion of sewage dumped in this region. The vertical distribution showed that the coprostanol concentration was the highest in the top 5-cm layer near the Korea Strait, close to one of the two dumping sites. A high coprostanol concentration was also found near the coast further north, where the content peaked at ∼10 cm depth. The vertical distribution of coprostanol helped to estimate the sediment accumulation rate at sites where radiocarbon gradient was too small or the values were too variable.

Shallow-water whale-fall communities: Evidence from the middle–late Eocene basilosaurid whale bones, Wadi El-Hitan, Fayum, Egypt

The present study examines the basilosaurid whale bone specimens collected from the Sandouk El-Borneta section, Wadi El-Hitan (Valley of Whales), Fayum, Egypt. These specimens are embedded in highly fossiliferous calcareous sandstones of the middle Eocene Gehannam and the upper Eocene Birket Qarun formations. These whale bones display some post-mortem alterations, representing good signs of at least three distinct stages in what is called the whale fall. The co-occurrence of shark, ray, and crocodile fossil specimens with the examined whale bones may indicate the first mobile-scavenger stage. Borings of the siboglinid polychaete genus Osedax represent the most common worm type that deeply bioeroded the whale bones, contributing to their rapid degradation, and representing the enrichment-opportunist stage. These fossil traces of the bone-eating worm Osedax represent the first record in the Eocene Epoch regionally and inter-regionally, filling the gap between the Late Cretaceous and the Oligocene occurrences. Subsequently, the studied whale bones served as hard substrates for some calcareous tube-dwelling encrusting polychaetes, balanoid barnacles, sheet‐like cheilostome bryozoans, and scleractinian corals. Furthermore, a bioerosion structure produced by polychaete annelids was also observed. The presence of these sclerobionts assemblage confirms the well-developed final reef stage with prolonged exposure and colonisation of these whale bones prior to final burial. Based on the recorded post-mortem alterations, together with other sedimentological and palaeontological data, the studied whale bones were deposited in a shallow open marine bay to sheltered gulf environments, which were characterised by low depositional energy, low to moderate rate of sedimentation, and high surface water productivity.

Dimethylmercury as a Source of Monomethylmercury in a Highly Productive Upwelling System.

Monomethylmercury (MMHg) is a neurotoxicant that biomagnifies in marine food webs, reaching high concentrations in apex predators. To predict changes in oceanic MMHg concentrations, it is important to quantify the sources and sinks of MMHg. Here, we study mercury speciation in the California Current System through cruise sampling and modeling. Previous work in the California Current System has found that upwelling transports mercury-enriched deep waters to productive surface waters. These upwelled waters originate within the California Undercurrent water mass and are subsequently advected as a surface water parcel to the California Current. Between the two major water masses, we find that compared to the California Current, the California Undercurrent contains elevated dissolved total mercury (THg) and dimethylmercury (DMHg) concentrations by 59 and 69%, respectively. We explain that these differences result from losses during advection, specifically scavenging of THg and DMHg demethylation. We calculate a net DMHg demethylation rate of 2.0 ± 1.1% d-1 and build an empirically constrained mass budget model to demonstrate that net DMHg demethylation accounts for 61% of surface MMHg sources. These findings illustrate that DMHg is a significant source of MMHg in this region, challenging the current understanding of the major sources of marine MMHg.

Alternate use of fresh and saline waters to improve water productivity and soil sustainability of drip irrigated wheat in a semi-arid region

Severe problems of rising groundwater and secondary soil salinization demand holistic agricultural water management in canal command areas. Drip irrigation with alternate use of fresh and saline waters is an excellent irrigation approach to mitigate salinity stress caused by highly saline ground water and also to avoid soil sodicity hazards from canal water. Therefore, a study on cyclic mode conjunctive use of good quality surface water (SW) and highly saline ground water (GW) having average electric conductivity 0.3 and 7.6 dS m−1, respectively was carried out in wheat with five treatments of drip viz. 1SW1GW (1:1 :: SW:GW), 1SW2GW (1:2 :: SW:GW), 2SW1GW (2:1 :: SW:GW), SW (all SW), GW (all GW), and one treatment of conventional border ­irrigation method (BI). Highest grain yield was observed in SW (2.99 t ha−1) in 2017–2018 and BI (3.83 t ha−1) in 2018–2019 season. However, yield of SW and BI were statistically at par in both seasons. Grain yield of 1SW1GW, in which only 50% freshwater was used was found statistically at par with SW and BI. Physical Water Productivity was highest for SW, amounting 9.34 and 14.13 kg ha−1.mm−1 during 2017–2018 and 2018–2019, respectively. Soil salinity build up was highest for GW and lowest for SW, whereas it was almost similar for BI and 1SW1GW. Thus, 1SW1GW showed added benefits as its grain yield was statistically at par with SW and BI along with 50% fresh water saving over SW and higher water productivity than BI for similar salinity levels.

Paleoceanographic Significance of Calcareous Nannofossil Assemblages in the Tropic Shale of Utah during Oceanic Anoxic Event 2 at the Cenomanian/Turonian Boundary

Oceanic Anoxic Event 2 (OAE2) at the Cenomanian/Turonian Boundary (CTB: 93.9Ma) involved the global deposition of organic carbon-rich sediments, a distinctive positive shift in carbon isotope values, and significant species turnover, including changes in calcareous nannofossil assemblages. While it is thought that volcanism triggered organic C-rich sediment deposition during OAE2, it is unclear whether enhanced productivity, increased stratification, of some combination of the two increased organic matter preservation. Calcareous nannofossil assemblages have the potential to qualitatively assess changes in ocean nutrient and temperature conditions to disentangle such ecological dynamics during OAE2. Here we study an expanded section of the Tropic Shale in a drill core in southern Utah near the western margin of the Western Interior Seaway (WIS) to understand how circulation changed during the event and how this may have influenced primary productivity and organic carbon burial. Relative abundance data of well-preserved nannoplankton are complemented with measurements of trace metal, and organic carbon and carbonate concentrations to determine changes in temperature and water column structure, as well as controls on surface water productivity. Detailed statistical analysis helps refine species paleoecologies combined with information from planktic and benthic foraminiferal assemblages and organic biomarkers. Changes in calcareous nannofossil assemblages indicate that near the start of OAE2 the western WIS surface ocean actually cooled for a short time. Following this, surface waters became warmer and more stratified as a Tethyan water mass invaded the seaway. Assemblages suggest that warmth persisted for much of the OAE2 interval, while stratification waxed and waned. The local seaway cooled near the end of OAE2 as Boreal water masses streamed along the western margin. Variations, including the decrease in the abundance of Biscutum constans and short-lived peaks in the abundance of Eprolithus spp. are super regional or possibly global in extent. There is no correlation between calcareous nannofossil assemblages and trace metal concentrations, suggesting they were unaffected by volcanism-related nutrient inputs. Assemblages support other data that suggest increased stratification influenced organic carbon burial in the Western Interior Seaway, and possibly elsewhere, during OAE2.

Picoplanktonic methane production in eutrophic surface waters

Abstract. Over the past decade, extensive research has delved into the methane (CH4) paradox, which involves aerobic CH4 production. We present noteworthy observations of CH4 oversaturation within the surface layer of the central Chile upwelling zone (36° S, 73° W) over two consecutive seasonal cycles (2018–2021). Complementing these observations, CH4 cycling experiments were conducted, utilizing distinct plankton fractions (encompassing the natural planktonic community, fractions < 150, < 3 and < 0.2 µm), in different productivity periods of phytoplanktonic production and composition throughout the year. Our findings underscore the pivotal role of picoplankton (< 3 µm) in CH4 production on the ocean surface, contrasting with the limited contribution of larger microorganisms (< 150 µm). Notably, incubations with methylated substrates, such as methylphosphonic acid (MPn) and trimethylamine (TMA), induce heightened CH4 production within the picoplanktonic fraction. This phenomenon is consistently observed during both upwelling (austral spring–summer) and non-upwelling (winter) seasons, with significance in the latter period, when Synechococcus sp. exhibits notably high relative abundance. Long-term microcosm experiments highlight the crucial roles played by heterotrophic bacteria and cyanobacteria in methylotrophic methanogenesis. This process enhances CH4 production, facilitated by the recycling of dissolved organic carbon (DOC). Picoplankton emerges as a pivotal factor influencing the recycling of methylated substrates, and it is responsible for maintaining CH4 supersaturation. These findings provide valuable insights into the biogeochemical processes driving CH4 dynamics, particularly in highly productive upwelling areas.

Taphonomy of the middle Miocene regular echinoid spines from Cairo-Suez District, Egypt: Palaeoecological and palaeoenvironmental interpretations

The palaeoecology and palaeoenvironment of a dense cluster of regular echinoid spines that occur in an oyster-rich limestone layer are interpreted based on detailed taphonomic study. The paucispecific macrobenthic assemblage associated with these echinoid spines have been studied in the middle Miocene Geniefa Formation, Gebel Gharra, Cairo-Suez District of Egypt. Both echinoid spines and macrobenthic assemblages are dominated by moderately to well-preserved, moderately packed, highly disarticulated, moderately to poorly sorted, re-oriented, and moderately to highly fragmented specimens, confirming parautochthonous nature of this assemblage. The spines are commonly encrusted and bored. At least four taxonomic groups of encrusters are identified, including juvenile oysters, sheet-like cheilostome bryozoans, serpulids, and balanoid barnacles. The diversity of bioerosional ichnotaxa is moderate and includes Gastrochaenolites, Entobia, Maeandropolydora, Trypanites, and Spirolites. Moreover, some echinoid spines display distinct biting traces of fish, representing the first recorded of fish tooth bite marks on remains of regular echinoid spines from the Miocene deposits of Egypt. The faunal composition and trophic structure of the studied faunal assemblage indicate fully oxygenated and euhaline shallow-marine environment with meso- to eutrophic productivity level. The controlling environmental parameters include substrate consistency, bathymetry, water energy, surface-water productivity, and rate of sedimentation. Furthermore, two scenarios for sclerobionts colonisation and development of fish bite marks have been proposed. In the first syn vivo scenario, predatory fish either focused on the encrusting organisms attached to the spines or attacked the echinoid as a food source, resulting in the separation of spines from their original test during the echinoid’s life. In the second post-mortem scenario, disarticulated spines serve as a hard substrate for the colonisation of sclerobionts. Once again, the spines became subject to attacks by fish that fed on the encrusters, resulting in additional bite marks.

Organic matter accumulation in response to tectonism: New data from the Upper Permian Dalong Formation black shales in the Western Hubei Basin, South China and its implications for the end-Permian mass extinction

Redox conditions and primary productivity are proposed to be the major controls of organic matter accumulation in sediments. Studies of the impacts of tectonic processes on organic matter accumulation are still limited. We took the Late Permian-Early Triassic Western Hubei Basin, located in the north of South China, as an example. In black shales in the Dalong Formation, bottom-water redox conditions and surface-water productivity are constrained as shown by the analysis of total organic carbon (TOC) as well as geochemical (enrichment of Mo, U, Ni, Cd and Zn) and mineralogical characteristics. The highest productivity is present in the lower Dalong Formation, and the most intensively anoxic and restricted conditions are found in the middle Dalong Formation. The correlation of the sedimentary successions on the two sides of the basin's boundary fault indicates a subsidence of the basement during the deposition of the lower-middle Dalong Formation, followed by an uplift during the deposition of the upper Dalong Formation. When looked at in conjunction with the tectonic background of the basin, the new data in the present study support that: (1) the collision of the South and North China plates controlled the paleoceanographic and hydrographic evolution of the Western Hubei Basin, as well as organic matter accumulation in sediments; (2) upwelling-induced high productivity and good preservation in an anoxic environment jointly contributed to organic matter enrichment in the lower Dalong Formation, while good preservation in an anoxic environment was the major cause of organic matter enrichment in the middle-upper Dalong Formation. This study indicates that tectonism can act as the ultimate drive mechanism for organic matter accumulation in black shales. Our data also suggest that plate tectonism in related to the assembly of East Asian blocks with the main body of Pangea can also be treated as one of the ultimate drive mechanisms of the end-Permian mass extinction as it created the restricted, anoxic Paleo-Tethys Ocean, which greatly disturbed- global seawater circulation.

Iron “ore” nothing: benthic iron fluxes from the oxygen-deficient Santa Barbara Basin enhance phytoplankton productivity in surface waters

Abstract. The trace metal iron (Fe) is an essential micronutrient that controls phytoplankton productivity, which subsequently affects organic matter cycling with feedback on the cycling of macronutrients. Along the continental margin of the US West Coast, high benthic Fe release has been documented, in particular from deep anoxic basins in the Southern California Borderland. However, the influence of this Fe release on surface primary production remains poorly understood. In the present study from the Santa Barbara Basin, in situ benthic Fe fluxes were determined along a transect from shallow to deep sites in the basin. Fluxes ranged between 0.23 and 4.9 mmol m−2 d−1, representing some of the highest benthic Fe fluxes reported to date. To investigate the influence of benthic Fe release from the oxygen-deficient deep basin on surface phytoplankton production, we combined benthic flux measurements with numerical simulations using the Regional Ocean Modeling System coupled to the Biogeochemical Elemental Cycling (ROMS-BEC) model. For this purpose, we updated the model Fe flux parameterization to include the new benthic flux measurements from the Santa Barbara Basin. Our simulations suggest that benthic Fe fluxes enhance surface primary production, supporting a positive feedback on benthic Fe release by decreasing oxygen in bottom waters. However, a reduction in phytoplankton Fe limitation by enhanced benthic fluxes near the coast may be partially compensated for by increased nitrogen limitation further offshore, limiting the efficacy of this positive feedback.

Changes in the Red Sea overturning circulation during Marine Isotope Stage 3

Abstract. The oceanography of the Red Sea is controlled by the restricted exchange of water masses with the Indian Ocean and by high evaporation rates due to the arid climate of the surrounding land areas. In the northern Red Sea, the formation of oxygen-rich subsurface water ventilates the deeper parts of the basin, but little is known about the variability in this process in the past. The stable oxygen and carbon isotope records of epibenthic foraminifera from a sediment core of the central Red Sea and comparison with existing isotope records allow for the reconstruction of changes in the Red Sea overturning circulation (ROC) during Marine Isotope Stage 3. The isotope records imply millennial-scale variations in the ROC, in phase with the climate variability in the high northern latitudes. This suggests an immediate response of dense-water formation to the regional climate and hydrology of the northern Red Sea. Deep-water formation was intensified under the influence of cold and hyper-arid conditions during Heinrich stadials and was diminished during Dansgaard–Oeschger interstadials. While these changes are reflected in both stable oxygen and carbon isotope records, the latter data also exhibit changes in phase with the African–Indian monsoon system. The decoupling of the stable carbon and oxygen isotope records at the summer monsoon maximum centered around 55–60 ka may be associated with an increased inflow of nutrient-rich intermediate waters from the Arabian Sea to the central Red Sea. This process fueled local surface water productivity, resulting in enhanced remineralization of sinking organic matter and release of 12C at intermediate water depths.

Pulsed inputs of high molecular weight organic matter shift the mechanisms of substrate utilisation in marine bacterial communities.

Heterotrophic bacteria hydrolyze high molecular weight (HMW) organic matter extracellularly prior to uptake, resulting in diffusive loss of hydrolysis products. An alternative 'selfish' uptake mechanism that minimises this loss has recently been found to be common in the ocean. We investigated how HMW organic matter addition affects these two processing mechanisms in surface and bottom waters at three stations in the North Atlantic Ocean. A pulse of HMW organic matter increased cell numbers, as well as the rate and spectrum of extracellular enzymatic activities at both depths. The effects on selfish uptake were more differentiated: in Gulf Stream surface waters and productive surface waters south of Newfoundland, selfish uptake of structurally simple polysaccharides increased upon HMW organic matter addition. The number of selfish bacteria taking up structurally complex polysaccharides, however, was largely unchanged. In contrast, in the oligotrophic North Atlantic gyre, despite high external hydrolysis rates, the number of selfish bacteria was unchanged, irrespective of polysaccharide structure. In deep bottom waters (> 4000 m), structurally complex substrates were processed only by selfish bacteria. Mechanisms of substrate processing-and the extent to which hydrolysis products are released to the external environment-depend on substrate structural complexity and the resident bacterial community.

Intrashelf basin record of redox and productivity changes along the Arabian margin of Neo-Tethys during Oceanic Anoxic Event 1a

The biotic, environmental, climatic, oceanic, and sea-level perturbations during the Early Aptian Oceanic Anoxic Event (OAE) 1a have been extensively documented from both deep- and shallow-marine deposits worldwide. However, there has been relatively little comparative assessment of the simultaneous interplay among organic carbon burial, redox conditions, terrigenous output, and productivity, leading to a lack of precise constraints on these relationships. Here, we use analyses of stable carbon isotopes (δ13Corg, δ13Ccarb, and Δ13C), total organic carbon (TOC), detrital proxies (Al, Si, Ti, K), redox-sensitive (RSTE: U, V, Mo) and productive-sensitive (PSTE: P, Cu, Ni) trace elements from a continuous, predominantly carbonate succession of the Kazhdumi Intrashelf Basin to evaluate the culprits for the OAE1a-associated changes in bottom-water oxygenation, organic-rich layer formation, and biotic shifts along the Arabian margin of the Neo-Tethys. Concentrations of Al-normalized RSTE and TOC values indicate that the bottom water conditions ranged from oxic prior to and at the onset of the OAE 1a (carbon-isotope segments C2 to basalmost C4 sensu Menegatti et al., 1998), to anoxic-suboxic but not euxinic (Mo < 25 ppm) during the lower C4 through C5 + C6 segments, and then returned to oxic-suboxic in the remaining C5 + C6 segment. The increase in Al-normalized PSTE coupled with TOC concentrations in the basal C4 is coeval with a change from predominantly orbitolinid-ostreid to planktic foraminifera-radiolarian biota. The periodically high productivity, driven both by the surface-water productivity as well as by phosphorus recycling from the sediments, continued through the C5 + C6 segments as evidenced by matching Si/Al and PSTE peaks (Cu/Al and Ni/Al). The study sheds new light on the causes of variations in bottom-water deoxygenation, organic content, nutrient availability, and biotic shifts in semi-restricted, relatively deep (>100 m), continental-margin basins during major oceanic perturbations.

Origin and alteration of sinking and resuspended organic matter on a benthic nepheloid layer influenced continental shelf

Understanding the biogeochemical transformations that particulate organic matter (POM) undergoes from production in surface waters to deposition on the seafloor is fundamental for interpreting the paleoceanographic record preserved in sediments. Here we use compound-specific N isotope analysis of amino acids (δ15NAA) from two sediment traps (deployed at 150 m and 250 m water depth) and underlying push-core sediments to investigate temporal and depth related changes in the export flux on a continental shelf basin influenced by a prominent benthic nepheloid layer (BNL). Despite apparent isotopic similarities between traps, molecular data show distinct patterns between the shallower (150 m) and deeper (250 m) sediment traps – specifically, depth-related differences in the relative contributions of particles with different processing histories. The export flux in the shallower trap is dominated by animal sources rather than phytodetritus. Significant microbial degradation in the deeper trap and sediments emphasizes the key roles of resuspension-deposition cycles that are mainly (but not exclusively) facilitated by a thick year-around BNL in the region. The BNL acts as a ‘microbial reactor’ reworking sinking POM before its ultimate deposition, and also diluting exported material with previously deposited surface sediments. This heavy modification of POM during its passage through and residence in the BNL opens potentially important questions for our current understanding of OC preservation and interpretations of proxy δ15N records in BNL-influenced areas.

Dynamic evolution of marine productivity, redox, and biogeochemical cycling track local and global controls on Cryogenian sea-level change

The Cryogenian period of the Neoproterozoic was marked by two Snowball Earth glaciations that bookended a non-glacial interval that lasted roughly 10 million years. The break-up of Rodinia in the early Neoproterozoic gave rise to numerous rift basins whose time-varying redox dynamics, linked to marine productivity and biogeochemical cycling, were likely controlled by varying degrees of basin restriction in response to sea-level change. Those drivers were associated with both global glaciations (Sturtian deglaciation and the possibly early onset of the Marinoan) and local tectonics (regional basin uplift/subsidence). To explore these relationships, we focus on the Datangpo Formation, an exceptional shale sequence deposited in a marginal rift basin in South China. The Datangpo succession comprises lower organic-rich black shales and upper organic-lean gray shales/siltstones. As elucidated by our integrated data, the former were deposited in H2S-rich, deep-water settings underlying highly productive surface waters. We attribute this productivity to elevated nutrient supplies replenished from the open ocean following a sea-level rise, as augmented by high terrestrial input of phosphate in the wake of the Sturtian deglaciation. By contrast, the gray shales/siltstones were associated with relatively shallow deposition in an oxic/suboxic water column with suppressed primary production. The major driver of this dramatic shift in water chemistry may lie with a greater extent of basin isolation and the corresponding reduction in nutrient fluxes from the global ocean due to a significant sea-level fall—possibly linked to regional basin uplift. At this time, the basin experienced an elevated influx of detrital clays and organic material, perhaps triggered by lower sea level. Surprisingly, we found that euxinic conditions prevailed in the water column for a substantial period following the onset of more isolated conditions. This relationship can be attributed to redox-dependent phosphorus cycling. Specifically, H2S-replete conditions facilitated regeneration of bioavailable phosphorus, initiating a positive feedback that allowed the persistence of anoxic waters. This study advances our understanding of nutrient cycling and biogeochemical evolution during the Cryogenian under the influence of large-scale sea-level fluctuations and associated time-varying connectivity to the open ocean that must have been common in many basins at this time. Furthermore, our study sheds novel light on the iron proxy and early sulfur cycling. Almost all of the terrestrial silicate-bound iron was likely converted into highly reactive phases due to strong chemical weathering, while sulfur data revealed isotopic stratification in the wake of Sturtian deglaciation, with important messages for interpretations of analogous data in the aftermath of other climate upheavals in Earth history.

Hypoxia-tolerant zooplankton may reduce biological carbon pump efficiency in the Humboldt current system off Peru

In the ocean, downward flux of particles produced in sunlit surface waters is the major component of the biological carbon pump, which sequesters atmospheric carbon dioxide and fuels deep-sea ecosystems. The efficiency of downward carbon transfer is expected to be particularly high in tropical upwelling systems where hypoxia occurring beneath the productive surface waters is thought to hamper particle consumption. However, observations of both particle feeders and carbon export in low-oxygen waters are scarce. Here, we provide evidence that hypoxia-tolerant zooplankton feed on sinking particles in the extensive Oxygen Minimum Zone (OMZ) off Peru. Using several arrays of drifting sediment traps and in situ imaging, we show geochemical and morphological transformations of sinking particles and substantial control of carbon export by zooplankton. Our findings challenge the assumption of a consistently efficient biological carbon pump in OMZs and further demonstrate the need to consider mesopelagic organisms when studying oceanic carbon sequestration.