Deep-sea Sites Research Articles

Planktic foraminifer response and paleoceanographic changes across the Danian – Selandian interval in the southern Tethys (Elles section, Central Tunisia)

The Danian – Selandian interval experienced several paleoceanographic changes with two prominent carbon cycle perturbations: the Dan-C2 and Latest Danian Event (LDE). We present integrated planktic foraminifer biostratigraphy, carbon isotope stratigraphy, and paleoceanographic interpretations from the Danian-Selandian at Elles, central Tunisia. Whereas the response of the planktic foraminifers to these events has been widely explored in deep sea sites, this shallow Tethyan shelf setting is significant for evaluating the expression of these global events in proximal environments. Both the Dan-C2 and LDE events are distinguished in our δ13Cbenthic record by biostratigraphically-constrained negative carbon isotope excursions. Recognition of these reliably dated excursions enables assessment of bioevent synchroneity. The lowest occurrence of Subbotina triloculinoides was diachronous at Dan-C2, but the highest occurrence of Praemurica was broadly synchronous at the onset of the LDE. Subsequent lowest occurrence of Igorina albeari shows latitudinal diachroneity between ocean basins. The planktic foraminifer assemblage composition indicates mesotrophic and stratified conditions in the early Danian, warming and increasing oligotrophy during the P2 Zone, peak oligotrophy and enhanced stratification across the LDE, shallowing-induced disruption of the upper water column niches in the latest Danian, and recovery with high diversity in the early Selandian. The Dan-C2 excursion shows a small 0.3‰ negative δ13C shift coinciding with increased abundance of low oxygen tolerant biserial Chiloguembelina, suggesting slightly stressed conditions. In contrast, the larger 0.6‰ LDE excursion coincides with the extinction of Praemurica and proliferation of Morozovella, reflecting an oligotrophic surge with warming in the surface ocean.

Microbial decomposition of biodegradable plastics on the deep-sea floor

Microbes can decompose biodegradable plastics on land, rivers and seashore. However, it is unclear whether deep-sea microbes can degrade biodegradable plastics in the extreme environmental conditions of the seafloor. Here, we report microbial decomposition of representative biodegradable plastics (polyhydroxyalkanoates, biodegradable polyesters, and polysaccharide esters) at diverse deep-sea floor locations ranging in depth from 757 to 5552 m. The degradation of samples was evaluated in terms of weight loss, reduction in material thickness, and surface morphological changes. Poly(l-lactic acid) did not degrade at either shore or deep-sea sites, while other biodegradable polyesters, polyhydroxyalkanoates, and polysaccharide esters were degraded. The rate of degradation slowed with water depth. We analysed the plastic-associated microbial communities by 16S rRNA gene amplicon sequencing and metagenomics. Several dominant microorganisms carried genes potentially encoding plastic-degrading enzymes such as polyhydroxyalkanoate depolymerases and cutinases/polyesterases. Analysis of available metagenomic datasets indicated that these microorganisms are present in other deep-sea locations. Our results confirm that biodegradable plastics can be degraded by the action of microorganisms on the deep-sea floor, although with much less efficiency than in coastal settings.

Environmental heterogeneity of cold seep by biological trait analysis of marine nematodes at Site F cold seep in South China Sea

Cold seeps provide high environmental heterogeneity for marine benthos. Site F is one of the active cold seeps in the South China Sea. In this study, free-living marine nematode communities were investigated at Site F and the adjacent deep-sea area. A total of 67 genera and 32 families were identified. The mean density at cold seep sites ranged from 13.6 to 181.8 ind./10 cm2, and that at the adjacent deep-sea sites ranged from 36.9 to 301.4 ind./10 cm2. At cold seep sites, the most dominant nematode genera were Desmoscolex, Pierrickia, Sabatieria, Halalaimus, and Dorylaimopsis while at deep-sea sites, the most dominant genera were Retrotheristus, Thalassomonhystera, Desmoscolex, Cobbia, and Halalaimus. Deposit feeders of nematodes were dominant at all sites. Results of biological trait analysis showed that there was high environmental heterogeneity for nematodes at Site F. Water depth, sediment organic matter content, and sand proportion had important influences on nematode communities.

Amino acid racemization in Neogloboquadrina pachyderma and Cibicidoides wuellerstorfi from the Arctic Ocean and its implications for age models

Abstract. We report the results of amino acid racemization (AAR) analyses of aspartic acid (Asp) and glutamic acid (Glu) in the planktic Neogloboquadrina pachyderma, and the benthic Cibicidoides wuellerstorfi, foraminifera species collected from sediment cores from the Arctic Ocean. The cores were retrieved at various deep-sea sites of the Arctic, which cover a large geographical area from the Greenland and Iceland seas (GIS) to the Alpha and Lomonosov ridges in the central Arctic Ocean. Age models for the investigated sediments were developed by multiple dating and correlation techniques, including oxygen isotope stratigraphy, magnetostratigraphy, biostratigraphy, lithostratigraphy, and cyclostratigraphy. The extent of racemization (D/L values) was determined on 95 samples (1028 subsamples) and shows a progressive increase downcore for both foraminifera species. Differences in the rates of racemization between the species were established by analysing specimens of both species from the same stratigraphic levels (n=21). Aspartic acid (Asp) and glutamic acid (Glu) racemize on average 16 ± 2 % and 23 ± 3 % faster, respectively, in C. wuellerstorfi than in N. pachyderma. The D/L values increase with sample age in nearly all cases, with a trend that follows a simple power function. Scatter around least-squares regression fits are larger for samples from the central Arctic Ocean than for those from the Nordic Seas. Calibrating the rate of racemization in C. wuellerstorfi using independently dated samples from the Greenland and Iceland seas for the past 400 ka enables estimation of sample ages from the central Arctic Ocean, where bottom water temperatures are presently relatively similar. The resulting ages are older than expected when considering the existing age models for the central Arctic Ocean cores. These results confirm that the differences are not due to taxonomic effects on AAR and further warrant a critical evaluation of existing Arctic Ocean age models. A better understanding of temperature histories at the investigated sites, and other environmental factors that may influence racemization rates in central Arctic Ocean sediments, is also needed.

Biogenic silica cycling in the Skagerrak

Dissolved silicate (H4SiO4) is essential for the formation of the opaline skeletal structures of diatoms and other siliceous plankton. A fraction of particulate biogenic silica (bSi) formed in surface waters sinks to the seabed, where it either dissolves and returns to the water column or is permanently buried. Global silica budgets are still poorly constrained since data on benthic bSi cycling are lacking, especially on continental margins. This study describes benthic bSi cycling in the Skagerrak, a sedimentary depocenter for particles from the North Sea. Biogenic silica burial fluxes, benthic H4SiO4 fluxes to the water column and bSi burial efficiencies are reported for nine stations by evaluating data from in-situ benthic landers and sediment cores with a diagenetic reaction-transport model. The model simulates bSi contents and H4SiO4 concentrations at all sites using a novel power law to describe bSi dissolution kinetics with a small number of adjustable parameters. Our results show that, on average, 1100 mmol m-2 yr-1 of bSi rains down to the Skagerrak basin seafloor, of which 50% is released back to overlying waters, with the remainder being buried. Biogenic silica cycling in the Skagerrak is generally consistent with previously reported global trends, showing higher Si fluxes and burial efficiencies than deep-sea sites and similar values compared to other continental margins. A significant finding of this work is a molar bSi-to-organic carbon burial ratio of 0.22 in Skagerrak sediments, which is distinctively lower compared to other continental margins. We suggest that the continuous dissolution of bSi in suspended sediments transported over long distances from the North Sea leads to the apparent decoupling between bSi and organic carbon in Skagerrak sediments.

Spatiotemporal dynamics of benthic bacterial communities in the Perdido Fold Belt, Northwestern Gulf of Mexico

This study analyzed the spatiotemporal dynamics of surficial benthic microbial communities in a bathymetric gradient (44 - 3573 m) across four oceanographic campaigns at the Perdido Fold Belt (PFB) in the northwestern Gulf of Mexico (nwGoM). Bioinformatic analysis of 16S rRNA gene amplicons grouped the 27 samples into three clusters according to a longitudinal bathymetric gradient. Differences in community structure among clusters, based on PERMANOVA analysis, were partially explained by cruise, water depth, temperature, salinity, nitrate plus nitrite, silicate, redox potential, Ni, Cd, Pb, and Al, as well by aliphatic and aromatic hydrocarbon concentrations. Into microbial community composition, Gemmatimonadaceae, Planctomycetaceae, and the JTB255 were detected at all depths across the four campaigns. Members of Anaerolinaceae and specific sulfate-reducing bacteria were more abundant in sites located between 43 and 1200 m, and Rhodospirillaceae, wb1-A12, OM1 clade, Desulfurellaceae, Gemmatimonadetes, Nitrospinaceae, and Clostridiaceae 1 were better represented in deeper sites. Alpha diversity was similar between the three groups and remained stable; however, 10 samples presented changes in the community structure across the four campaigns. Finally, a multivariable association analysis revealed 25 bacterial genera positively related with physicochemical parameters that characterized the environment from shallow to deep sea sites. Taken together, these results yield insights into the temporal stability of 17 of 27 sites in the PFB and revealed signature taxa with putatively ecological relevance in sedimentary environments.

The meiofauna of the Eratosthenes Seamount (eastern Mediterranean Sea)—first insights into taxa composition, distribution, and diversity

During the research cruise MSM 14/1 of RV M.S. MERIAN to the eastern Mediterranean Eratosthenes Seamount in 2009/2010, samples were taken with a multiple corer to get first insights into the composition, abundance, and diversity of the meiofauna. Along two transects, a north–south and a west–east ones, 14 sites were sampled on the seamount. Additionally, a distant deep-sea site provided meiobenthic material for comparison with that of the seamount. Altogether, 15 meiobenthic major taxa were found. Nematoda and Copepoda Harpacticoida strongly dominated all sites, followed by Annelida and Tardigrada at most sites. Although direct comparison pointed towards a northern seamount community, this could not be confirmed by a detailed community analysis regarding taxa composition, abundance expressed by density (ind./10 cm2), and taxa diversity. It revealed neither a homogeneous seamount community nor any faunistic relation with bathymetric or geographic gradients, with exception of the taxa diversity that apparently followed a latitudinal gradient. Generally, Eratosthenes Seamount presents a quite heterogeneous meiobenthic assemblage that may be linked to small-scale biotic and abiotic variables. Moreover, also the distant reference site did not differ significantly from the seamount sites. A brief comparison with other Atlantic and Mediterranean seamounts and islands indicates that Eratosthenes Seamount is not characterized by an impoverished meiobenthic fauna.

Reactive-transport modeling of neodymium and its radiogenic isotope in deep-sea sediments: The roles of authigenesis, marine silicate weathering and reverse weathering

Dissolved Rare Earth Elements (REE) and radiogenic neodymium (Nd) isotope composition (εNd) of seawater are widely used geochemical tools in studying marine processes, but their modern ocean budgets are poorly understood. Recent discoveries of large benthic fluxes of REE with unique εNd signatures from marine sediments, particularly in the deep-sea, have led to a “bottom-up” hypothesis, which suggests that early diagenesis below the sediment-water interface (SWI) controls the ocean's REE and εNd budgets. To investigate such sedimentary processes, we created a reactive-transport model for the biogeochemical cycling of Nd and εNd in marine sediments. Here, we attempt to quantify the roles of authigenesis, marine silicate weathering and reverse weathering in the diagenetic cycling of Nd and εNd at a deep-sea (3000 m) site on the Oregon margin.Our model predicts that, at this site, Nd carried by Fe/Mn oxides into sediments eventually transforms to authigenic Nd-phosphate, during which ∼9% of the incoming solid Nd flux is released as a dissolved benthic flux back to the overlying bottom water. We also find that the classic reversible scavenging formulation applied to Nd co-cycling with Fe/Mn oxides is inconsistent with the data. Rather, a co-precipitation formulation, assuming Nd is structurally incorporated into Fe/Mn oxides, successfully simulates the data. The model also shows that authigenesis alone cannot explain the pore water and authigenic εNd, which are both more radiogenic than bottom water at this site. However, the weathering of volcanic silicates sourced from the local subduction zone can successfully explain εNd. We suggest that, because reverse weathering by authigenic clay formation maintains the under-saturation of primary silicates in pore water, marine silicate weathering can proceed. The processes we model likely affect the sedimentary cycling of many other trace elements and isotopes, with much broader implications for the understanding of ocean biogeochemistry.

Meiofaunal nematode abundance, composition, and diversity at bathyal to hadal depths in the Southeast Pacific Ocean

Seamounts have long been hypothesized to be biodiversity hotspots for benthic organisms, but there is limited empirical evidence to support that notion. The present study sampled meiofauna from different deep-sea sites in the Southeast Pacific Ocean (seamounts, abyssal plain, trench, island) to assess causative factors that influence meiofauna and nematode diversity and community composition. Evidence found in this study suggests that composition of meiofauna is influenced by biogeographic region and biogeochemical factors, and the high diversity that was observed at the majority of seamount sites may be related to intermediate levels of disturbance and/or variability in deposition of organic material. More studies are needed to understand the causes for why biodiversity of meiofauna is often high at seamounts, but it is plausible that seamounts are important biodiversity hotspots for nematodes and other meiofauna owing to the interaction between physical currents and erosion and variability in depositional regimes. This suggests that seamounts could be important sites for the conservation of benthic communities in the deep sea, and this should be considered in the development of marine management strategies.

Early diagenetic control on the enrichment and fractionation of rare earth elements in deep-sea sediments.

The rare earth elements and yttrium (REY) in bioapatite from deep-sea sediments are potential proxies for reconstructing paleoenvironmental conditions. However, the REY enrichment mechanism and the reliability of this tracer remain elusive because of the lack of key information from ambient pore water. Here, we report high-resolution geochemical data for pore water, bottom water, and bioapatite from deep-sea sites in the western Pacific. Our results reveal that the benthic flux of REY from the deep sea is less substantial than from the shallow marine realm, resulting in REY-rich sediment. The depth distribution of REY in pore water is opposite to that of bioapatite, and REY patterns and neodymium isotopic compositions are not uniformly distributed within bioapatite. These results indicate alteration of REY and neodymium isotopic compositions during early diagenesis. Therefore, we infer that REY from bioapatite are not robust recorders of the deep marine environment through Earth’s history.

Cosmopolitism, rareness and endemism in deep-sea marine nematodes

Nematodes represent the most abundant benthic metazoan of all seas and oceans, and their relative importance increases with increasing water depth. Understanding the biodiversity patterns of this dominant phylum could be a critical step towards our comprehension of the evolutionary patterns across the largest biome of the biosphere. For instance, it has been assumed for a long time that nematodes are ubiquitous across depths, latitudes and biogeographic regions, but there is still little scientific evidence for this lack of endemism. The present study is based on a meta-analysis of nematode biodiversity data collected from 246 deep-sea sites of the Atlantic Ocean and Mediterranean Sea. We explored the cosmopolitanism, rareness and potential endemism of nematode genera in deep-sea sediments. The results of this analysis indicate that only one-third of nematode families are widely distributed and could potentially be cosmopolitan, whereas 94% of the nematode genera are linked to specific habitats or bathymetric ranges. Singleton nematode genera (i.e. genera presenting as a single individual only in one specific habitat) increased in importance with increasing water depth. We conclude that rareness and endemism may be a far more common feature than previously thought in deep-sea nematode assemblages and hypothesise that the deep ocean interior could be a huge reservoir of endemic nematode species.

Extremophiles in Earth's Deep Seas: A View Toward Life in Exo-Oceans.

Humanity's search for extraterrestrial life is a modern manifestation of the exploratory and curious nature that has led us through millennia of scientific discoveries. With the ongoing exploration of extraterrestrial bodies, the potential for discovery of extraterrestrial life has expanded. We may better inform this search through an understanding of how life persists and flourishes on Earth in a myriad of environmental extremes. A significant proportion of our knowledge of extremophiles on Earth comes from studies on deep ocean life. Here, we review and synthesize the range of environmental extremes observed in the deep sea, the life that persists in these extreme conditions, and the biological adaptations utilized by these remarkable life-forms. We also review confirmed and predicted extraterrestrial oceans in our solar system and propose deep-sea sites that may serve as planetary field analog environments. We show that the clever ingenuity of evolution under deep-sea conditions suggests that the plausibility of extraterrestrial life is much greater than previously thought.

Diachroneity Rules the Mid-Latitudes: A Test Case Using Late Neogene Planktic Foraminifera across the Western Pacific

Planktic foraminifera are commonly used for first-order age control in deep-sea sediments from low-latitude regions based on a robust tropical–subtropical zonation scheme. Although multiple Neogene planktic foraminiferal biostratigraphic zonations for mid-latitude regions exist, quantification of diachroneity for the species used as datums to test paleobiogeographic patterns of origination and dispersal is lacking. Here, we update the age models for seven southwest-Pacific deep-sea sites using calcareous nannofossil and bolboform biostratigraphy and magnetostratigraphy, and use 11 sites between 37.9° N and 40.6° S in the western Pacific to correlate existing planktic foraminiferal biozonations and quantify the diachroneity of species used as datums. For the first time, northwest and southwest Pacific biozones are correlated and compared to the global tropical planktic foraminiferal biozonation. We find a high degree of diachroneity in the western Pacific, within and between the northwest and southwest regions, and between the western Pacific and the tropical zonation. Importantly, some datums that are found to be diachronous between regions have reduced diachroneity within regions. Much work remains to refine regional planktic foraminiferal biozonations and more fully understand diachroneity between the tropics and mid-latitudes. This study indicates that diachroneity is the rule for Late Neogene planktic foraminifera, rather than the exception, in mid-latitude regions.

Sediment-Derived Dissolved Organic Matter Stimulates Heterotrophic Prokaryotes Metabolic Activity in Overlying Deep Sea in the Ulleung Basin, East Sea

The effects of benthic dissolved organic carbon (DOC) flux on the dynamics of DOC in the deep continental margins (200 – 2000 m depth) is poorly understood. We investigated heterotrophic prokaryotes (hereafter bacteria) production (BP) and the bio-reactive properties of sediment-derived dissolved organic matter (SDOM) to elucidate microbially mediated cause-effect relationships regarding the rapid consumption of dissolved oxygen (DO) and accumulation of humic-like fluorescent DOM (FDOMH) in the deep-water column (750 – 2000 m depth range) of the Ulleung Basin (UB) in the East Sea. BP in the deep water (2.2 μmol C m-3 d-1) of the UB was among the highest reported for various deep-sea sites. The high DOC concentration (55 μM) likely supported the high BP seen in the deep-water column of the UB. Concentrations of DOC and C1 component of the FDOMH, which is indicative of microbial metabolic by-products, were 13-fold and 20-fold greater, respectively, in pore water than in the overlying bottom water, indicating that the sediment in the continental margins is a significant source of DOM in the overlying water column. Fine-scale water sampling revealed that BP near the sediment (0 – 30 m above the seafloor; 2.78 μmol C m-3 d-1) was 1.67 times higher than that measured in the water column above (30 – 100 m above the seafloor; 1.67 μmol C m-3 d-1). In addition, BP increased in the bottom water incubation amended with SDOM-containing pore water (PW). The results demonstrated that SDOM contains bio-reactive forms of DOM that stimulate heterotrophic microbial metabolism at the expense of oxygen in the bottom water layer. The accumulation of C1 component in both PW-amended and unamended bottom water incubation (i.e., without an extra DOM supply from sediment) further indicated that refractory DOM is produced autochthonously in the water column via heterotrophic metabolic activity. This explains in part the microbially mediated accumulation of excess FDOMH in the deep-water column of the UB. Overall results suggest that the benthic release of bio-reactive DOM may be of widespread significance in controlling microbial processes in the deep-water layer of marginal seas.

Local Environmental Conditions Promote High Turnover Diversity of Benthic Deep-Sea Fungi in the Ross Sea (Antarctica).

Fungi are a ubiquitous component of marine systems, but their quantitative relevance, biodiversity and ecological role in benthic deep-sea ecosystems remain largely unexplored. In this study, we investigated fungal abundance, diversity and assemblage composition in two benthic deep-sea sites of the Ross Sea (Southern Ocean, Antarctica), characterized by different environmental conditions (i.e., temperature, salinity, trophic availability). Our results indicate that fungal abundance (estimated as the number of 18S rDNA copies g−1) varied by almost one order of magnitude between the two benthic sites, consistently with changes in sediment characteristics and trophic availability. The highest fungal richness (in terms of Amplicon Sequence Variants−ASVs) was encountered in the sediments characterized by the highest organic matter content, indicating potential control of trophic availability on fungal diversity. The composition of fungal assemblages was highly diverse between sites and within each site (similarity less than 10%), suggesting that differences in environmental and ecological characteristics occurring even at a small spatial scale can promote high turnover diversity. Overall, this study provides new insights on the factors influencing the abundance and diversity of benthic deep-sea fungi inhabiting the Ross Sea, and also paves the way for a better understanding of the potential responses of benthic deep-sea fungi inhabiting Antarctic ecosystems in light of current and future climate changes.

The benthic foraminiferal response to the mid-Maastrichtian event in the NW-European chalk sea of the Maastrichtian type area

Abstract The mid-Maastrichtian carbon isotope event (MME), dated at ∼69 Ma, reflects a perturbation of the global carbon cycle that, in part, correlates with the enigmatic global extinction of ‘true’ (i.e., non-tegulated) inoceramid bivalves. The mechanisms of this extinction event are still debated. While both the inoceramid extirpation and MME have been recorded in a variety of deep-sea sites, little is known about their expression in epicontinental chalk seas. In order to study the shallow-marine signature of the MME in this epicontinental shelf sea, we have generated quantitative foraminiferal assemblage data for two quarries (Hallembaye, NE Belgium; ENCI, SE Netherlands) in the Maastrichtian type area, complemented by a species-specific benthic δ13C record. In contrast to deep-sea records, no significant changes in benthic foraminiferal assemblages and benthic foraminiferal accumulation rates are observed across the MME in the type-Maastrichtian area. At the Hallembaye quarry, the otherwise rare endobenthic species Cuneus trigona reaches a transient peak abundance of 33.3% at the onset of the MME, likely caused by a local transient change in organic matter flux to the seafloor. Nevertheless, high and near-constant species evenness shows that neither oxygen nor organic matter flux was limited across the extinction level or during the MME. Benthic foraminiferal data from the uppermost part of the studied section, above the MME, indicate a significant increase in food supply to the seafloor. Decreased amounts of terrigenous elements across this interval document a lesser riverine or aeolian influx, which means that the increased benthic productivity is linked to a different origin. Potentially, the continuous precipitation of chalk under nutrient-poor conditions in the Late Cretaceous chalk sea was enabled by efficient nutrient recycling in the water column. In shallower depositional settings, nutrient recycling took place closer to the seafloor, which allowed more organic matter to reach the bottom. These results provide insights in the importance of nutrient cycling for biological productivity in the NW-European chalk sea.

Sediment Resuspension and Associated Extracellular Enzyme Activities Measured ex situ: A Mechanism for Benthic-Pelagic Coupling in the Deep Gulf of Mexico

Sediment resuspension caused by near-bed currents mediates exchange processes between the seafloor and the overlying water column, known as benthic-pelagic coupling. To investigate the effects of sediment resuspension on microbial enzyme activities in bottom waters (<500 m), we conducted onboard erosion experiments using sediment cores taken with a multi-corer from six deep-sea sites in the northern Gulf of Mexico. We then incubated the core-top water with resuspended sediments in roller tanks to simulate bottom water conditions following sediment resuspension. Bacterial cell abundance, particulate organic matter content, and potential rates of three hydrolytic enzymes (leucine aminopeptidases – PEP; β-glucosidases – GLU, lipases – LIP) were monitored during the experimentally-generated erosion events and subsequently in the roller tanks to examine whether resuspension of deep-sea sediments enhances activities of extracellular enzymes in overlying waters. Surficial sediments were resuspended at critical shear stress velocities between 1.4 and 1.7 cm s–1, which parallel bottom water currents of 28 and 34 cm s–1. Only one of our nine cores resisted experimentally generated bottom shear stresses and remained undisturbed, possibly as a result of oil residues from natural hydrocarbon seeps at the investigated site. The most notable enzymatic responses to sediment resuspension were found for LIP activities that increased in overlying waters of all eight of our resuspended cores and remained at high levels during the roller tank incubations. PEP and GLU showed orders of magnitude lower rates and more variable responses to experimentally resuspended sediments compared with LIP. We also found a disconnect between enzyme activities and bacterial cell numbers, indicating a major role of extracellular enzymes physically disconnected from microbial cells in our experiments. Our results demonstrate that sediment resuspension may promote organic matter breakdown in bottom waters by supplying extracellular enzymes without requiring a bacterial growth response. The marked increase in LIP activity suggests that resuspended enzymes may affect the degradation of petroleum hydrocarbons, including those from the natural seeps that are abundant in the investigation area.

Evaluating sediment and water sampling methods for the estimation of deep-sea biodiversity using environmental DNA

Despite representing one of the largest biomes on earth, biodiversity of the deep seafloor is still poorly known. Environmental DNA metabarcoding offers prospects for fast inventories and surveys, yet requires standardized sampling approaches and careful choice of environmental substrate. Here, we aimed to optimize the genetic assessment of prokaryote (16S), protistan (18S V4), and metazoan (18S V1–V2, COI) communities, by evaluating sampling strategies for sediment and aboveground water, deployed simultaneously at one deep-sea site. For sediment, while size-class sorting through sieving had no significant effect on total detected alpha diversity and resolved similar taxonomic compositions at the phylum level for all markers studied, it effectively increased the detection of meiofauna phyla. For water, large volumes obtained from an in situ pump (~ 6000 L) detected significantly more metazoan diversity than 7.5 L collected in sampling boxes. However, the pump being limited by larger mesh sizes (> 20 µm), only captured a fraction of microbial diversity, while sampling boxes allowed access to the pico- and nanoplankton. More importantly, communities characterized by aboveground water samples significantly differed from those characterized by sediment, whatever volume used, and both sample types only shared between 3 and 8% of molecular units. Together, these results underline that sediment sieving may be recommended when targeting metazoans, and aboveground water does not represent an alternative to sediment sampling for inventories of benthic diversity.

Planktonic Foraminiferal Endemism at Southern High Latitudes Following the Terminal Cretaceous Extinction

AbstractAustral planktonic foraminiferal assemblages from immediately above the Cretaceous/Paleogene (K/Pg) boundary at Ocean Drilling Program Hole 690C (Maud Rise, Weddell Sea) and International Ocean Drilling Program Hole U1514C (southeast Indian Ocean) show a much different record of post-extinction recovery than anywhere outside the circum-Antarctic region. Species of Woodringina and Parvularugoglobigerina, genera with well-documented evolutionary successions within the early Danian P0 and Pα biozones at tropical/subtropical and mid-latitude localities, are absent from southern high latitude sequences. This study proposes new criteria for biostratigraphic correlation of the lowermost Danian Antarctic Paleocene AP0 and AP1 Zones using stratophenetic observations from Scanning Electron Microscope images of lower Danian planktonic foraminifera at deep-sea sites in the southern South Atlantic and southern Indian Ocean. The small but distinctive species Turborotalita nikolasi (Koutsoukos) is a highly reliable index species for the lowermost Danian as it consistently occurs immediately above the K/Pg boundary at multiple southern high latitude sites, which is consistent with its distribution at middle and low latitudes. Also useful for cross-latitude correlation is Parasubbotina neanika n. sp., which first appears within the lowermost Danian worldwide. The geographic distribution of the New Zealand species Antarcticella pauciloculata (Jenkins) and Zeauvigerina waiparaensis (Jenkins), as well as Eoglobigerina maudrisensis n. sp. from just above the K/Pg in the southern South Atlantic and southern Indian Ocean, helps define the extent of the Austral Biogeographic Province and provides evidence for marine communication via marine seaways across Antarctica. While An. pauciloculata was previously considered a benthic species, new stable isotope evidence demonstrates that it lived a planktonic mode of life. It is possible this species evolved from a benthic ancestor and that the benthic to planktonic transition occurred through an intermediate tychopelagic lifestyle at a time when calcareous plankton were less abundant as a result of the terminal Cretaceous mass extinction.

Shifts in Phytoplankton Composition and Stepwise Climate Change During the Middle Miocene

AbstractThe abundance and composition of modern phytoplankton are primarily related to equator‐to‐pole temperature gradients and global ocean circulation, which in turn determine the availability of nutrients in the photic zone. The nutricline is found at greater depths in warm, tropical waters, whereas more vigorous surface mixing in higher latitudes (seasonally) enhances nutrient availability and primary productivity. Ocean temperatures were ~7°C higher during the middle Miocene Climatic Optimum (MCO; ~16.9–14.7 million years ago, Ma), which was followed by Antarctic glaciation and global cooling during the middle Miocene Climate transition (MMCT; 14.7–13.8 Ma). Four decades ago, Haq (1980, https://doi.org.10.2307/1485353) already related migration patterns of low‐latitude versus high‐latitude calcareous nannoplankton in the Atlantic Ocean to major climatic fluctuations during the Miocene. Here, we detail and discuss the macroevolutionary patterns and processes across the middle Miocene (~16.5–11 Ma) at five deep sea sites on a north‐south transect in the Atlantic Ocean (57°N to 28°S). We show that the major cooling step toward the modern “icehouse” world impacted coccolithophore communities at all latitudes. Contrary to previous observations suggesting that tropical sites showed little change and that midlatitudes were the most sensitive recorders of climate change across the MMCT, we show that all sites recorded a marked diversification and increase in abundance of reticulofenestrids. Global cooling and related increased meridional overturning circulation are implicated as likely forcings for this macroevolutionary step toward establishing modern coccolithophore communities that are dominated by eurythermal and eurytrophic species such as Emiliania huxleyi.