Corrosive Bottom Waters Research Articles

Alkaline mineral addition to anoxic to hypoxic Baltic Sea sediments as a potentially efficient CO2-removal technique

Recent studies have begun to explore the potential of enhanced benthic weathering (EBW) in the Baltic Sea as a measure for climate change mitigation. To augment the understanding of EBW under seasonally changing conditions, this study aims to investigate weathering processes under anoxia to hypoxia in corrosive bottom waters, which reflect late summer conditions in the Baltic Sea. Dunite and calcite were added to sediment cores retrieved from Eckernförde Bay (Western Baltic Sea) with a constant flow-through of deoxygenated, CO2-enriched Baltic Sea bottom water. The addition of both materials increased benthic alkalinity release by 2.94 μmol cm−2 d−1 (calcite) and 1.12 μmol cm−2 d−1 (dunite), compared to the unamended control experiment. These excess fluxes are significantly higher than those obtained under winter conditions. The comparison with bottom water oxygen concentrations emphasizes that highest fluxes of alkalinity were associated with anoxic phases of the experiment. An increase in Ca and Si fluxes showed that the enhanced alkalinity fluxes could be attributed to calcite and dunite weathering. First order rate constants calculated based on these data were close to rates published in previous studies conducted under different conditions. This highlights the suitability of these proxies for mineral dissolution and justifies the use of these rate constants in modeling studies investigating EBW in the Baltic Sea and areas with similar chemical conditions. Generally stable pH profiles over the course of the experiment, together with the fact that the added minerals remained on the sediment surface, suggest that corrosive bottom waters were the main driving factor for the dissolution of the added minerals. These factors have important implications for the choice of mineral and timing for EBW as a possible marine carbon dioxide removal method in seasonally hypoxic to anoxic regions of the Baltic Sea.

Coastal freshening drives acidification state in Greenland fjords

Greenland's fjords and coastal waters are highly productive and sustain important fisheries. However, retreating glaciers and increasing meltwater are changing fjord circulation and biogeochemistry, which may threaten future productivity. The freshening of Greenland fjords caused by unprecedented melting of the Greenland Ice Sheet may alter carbonate chemistry in coastal waters, influencing CO2 uptake and causing biological consequences from acidification. However, few studies to date explore the current acidification state in Greenland coastal waters. Here we present the first-ever large-scale measurements of carbonate system parameters in 16 Greenlandic fjords and seek to identify the drivers of acidification state in these freshening ecosystems. Aragonite saturation state (Ω), a proxy for ocean acidification, was calculated from dissolved inorganic carbon (DIC) and total alkalinity from fjords along the east and west coast of Greenland spanning 68–75°N. Aragonite saturation was primarily >1 in the surface mixed layer. However, undersaturated—or corrosive––conditions (Ω < 1) were observed on both coasts (west: Ω = 0.28–3.11, east: Ω = 0.70–3.07), albeit at different depths. West Greenland fjords were largely corrosive at depth while undersaturation in East Greenland fjords was only observed in surface waters. This reflects a difference in the coastal boundary conditions and mechanisms driving acidification state. We suggest that advection of Sub Polar Mode Water and accumulation of DIC from organic matter decomposition drive corrosive conditions in the West, while freshwater alkalinity dilution drives acidification in the East. The presence of marine terminating glaciers also impacted local acidification states by influencing fjord circulation: upwelling driven by subglacial discharge brought corrosive bottom waters to shallower depths. Meanwhile, discharge from land terminating glaciers strengthened stratification and diluted alkalinity. Regardless of the drivers in each system, increasing freshwater discharge will likely lower carbonate saturation states and impact biotic and abiotic carbon uptake in the future.

Paleoceanographic Perturbations and the Marine Carbonate System during the Middle to Late Miocene Carbonate Crash—A Critical Review

This study intends to review and assess the middle to late Miocene Carbonate Crash (CC) events in the low to mid latitudes of the Pacific, Indian, Caribbean and Atlantic Oceans as part of the global paleoceanographic reorganisations between 12 and 9 Ma with an emphasis on record preservation and their relation to mass accumulation rates (MAR). In the Eastern Pacific the accumulation changes in carbonate and opal probably reflect an El-Niño-like state of low productivity, which marks the beginning of the CC-event (11.5 Ma), followed by decreased preservation and influx of corrosive bottom waters (10.3 to 10.1 Ma). At the same time in the Atlantic, carbonate preservation considerably increases, suggesting basin-to-basin fractionation. The low-latitude Indian Ocean, the Pacific and the Caribbean are all characterised by a similar timing of preservation increase starting at ~9.6–9.4 Ma, while their MARs show drastic changes with different timing of events. The Atlantic preservation pattern shows an increase as early as 11.5 Ma and becomes even better after 10.1 Ma. The shallow Indian Ocean (Mascarene plateau) is characterised by low carbonate accumulation throughout and increasing preservation after 9.4 Ma. At the same time, the preservation in the Atlantic, including the Caribbean, is increasing due to enhanced North Atlantic deep-water formation, leading to the increase in carbonate accumulation at 10 Ma. Moreover, the shoaling of the Central American Isthmus might have helped to enhance Caribbean preservation after 9.4 Ma. Lower nannoplankton productivity in the Atlantic should have additionally contributed to low mass accumulation rates during the late CC-interval. Overall, it can be inferred that these carbonate minima events during the Miocene may be the result of decreased surface ocean productivity and oceanographically driven increased seafloor dissolution.

Sources of Corrosive Bottom Water to Bellingham Bay, Washington State

The Salish Sea, spanning Washington State and British Columbia, receives relatively low pH water from upwelling in the northeast Pacific Ocean and the acidity of bottom water increases within the sea, particularly in winter. In order to better understand the processes that lead to bottom water acidification in this region, I quantified the sources of dissolved inorganic carbon (DIC) and alkalinity to bottom water in Bellingham Bay, a small embayment that is nevertheless representative of the broader Salish Sea. Corrosive bottom water with a pH of 7.5 and an aragonite saturation state of 0.51 was observed in deep water the northern portion of the bay. Water column respiration contributed 6.8 mmol DIC m−3 d−1 to bottom water while sediments added both DIC (13.6 mmol m−2 d−1) and alkalinity (9 mmol m−2 d−1). A two-layer box model assessed the relative importance of sources of corrosive water to the bay and calculated how bottom water pH and aragonite saturation state vary during spring. The 30-day model run calculated increases in bottom water DIC and alkalinity but decreases in bottom water pH and aragonite saturation state along with oscillations of these parameters with a period of approximately 6 days. A sensitivity analysis of the model demonstrated that water column respiration was the primary driver of corrosive bottom-water production. Sediment respiration contributed less to the production of corrosive water but was a source of alkalinity. The accumulation of corrosive bottom water in Bellingham Bay was also a function of mean circulation; the slowing of bottom water flow led to longer bottom-water residence time and an increase in bottom water corrosivity.

Early Paleogene biosiliceous sedimentation in the Atlantic Ocean: Testing the inorganic origin hypothesis for Paleocene and Eocene chert and porcellanite

The widespread occurrence of lower Eocene chert and porcellanite has been viewed as a major paleoceanographic issue since the advent of ocean drilling, and both biotic and abiotic forcings have been proposed to explain it. We present a reconstruction of indurated siliceous sediment (ISS) and preserved biosiliceous sediment (PBS) occurrences in the Atlantic Ocean through the Paleocene and Eocene (~66 through 34 Ma). ISS and PBS distributions reveal dissimilar temporal trends, with the peak of ISS occurrences coinciding with the Early Eocene Climatic Optimum, in line with previous studies. PBS occurrences show a generally increasing trend culminating between 44 and 43 Ma. The common co-occurrence of ISS and PBS, and their coherent geographic distribution lends strong support to the biogenic origin of the precursor to the widespread Paleogene ISS, and argues against an inorganic mode of early Cenozoic chert and porcellanite precipitation. Weight per cent biogenic opal records and trends in linear sedimentation rates indicate two plausible modes of silicification: 1) silicification due to prolonged exposure of biogenic opal-rich sediments to corrosive bottom waters; and 2) silicification due to elevated pressures and temperatures caused by rapid burial of biogenic opal-rich deposits. The confinement of ISS and PBS to proximal sites along continental margins points to the reliance of siliceous sedimentation through the Paleocene and Eocene on terrestrial supply of dissolved silicon. Consistent with this, quantitative siliceous microfossil assemblage records from the Blake Nose in the NW Atlantic indicate that the nutrient-rich marginal rather than oligotrophic pelagic settings hosted the majority of siliceous plankton production through the early Paleogene.The inorganic SiO2 precipitation model is unlikely to have been the dominant mechanism responsible for ubiquitous occurrences of early Paleogene ISS. We favor the biogenic ISS precursor scenario and reconcile it with the low-productivity early Cenozoic oceans by showing that large volumes of biogenic silica were supplied to the western North Atlantic Ocean from the North American margin through the Paleocene and Eocene. Dissolution of this surplus silica was facilitated by an early southwestward flow of young, SiO2-depleted waters from the North Atlantic. All these factors contributed to ISS and PBS focusing in the western North Atlantic through the early Paleogene.

Holocene chronostratigraphy of northeastern Baffin Bay based on radiocarbon and palaeomagnetic data

The northeastern Baffin Bay continental margin, which experiences high sediment accumulation rates, is an excellent location to study Holocene sedimentary variations. However, it is often difficult to obtain reliable chronologies of the sediment archives using traditional methods (d 18 O and radiocarbon) due to specific oceanographic conditions (e.g. corrosive bottom waters). Here we propose a chronostratigraphy of three cores collected on the northwestern Greenland margin (AMD14-204, AMD14-210 and AMD14-Kane2B) based on a combination of radiocarbon dating and palaeomagnetic records. Geophysical properties of discrete samples were used to verify the reliability of the palaeomagnetic records. Palaeomagnetic analyses indicate a strong and stable natural remanent magnetization carried by low coercivity ferrimagnetic minerals (magnetite) in the pseudo-single domain grain size range. Correlation of the full palaeomagnetic vector (inclination, declination, and relative palaeointensity) was used to establish a reliable chronostratigraphical framework for two of the cores (AMD14-204 and AMD14-210) and to propose an original palaeomagnetic record for the previously 14 C-dated core AMD14-Kane2B that covers most of the Holocene. Overall, this new chronostratigraphy allowed improvement of the timing of the main palaeoenvi-ronmental changes that occurred in this area during the Holocene.

Late Holocene oceanographic and climatic variability recorded by the Perseverance Drift, northwestern Weddell Sea, based on benthic foraminifera and diatoms

Abstract The Perseverance Drift, located in the Joinville-D' Urville Trough, northwestern Weddell Sea, records changes in ocean and sea ice conditions throughout the middle to late Holocene, with a record extending back to ca. 3400 yr BP. The 2562-cm composite record collected from a water depth of 806 m, documents the uppermost section of the 90-m thick sediment drift. Spring-blooming diatoms (Chaetoceros subg. Hyalochaete) are abundant through the sedimentary record. The greater proportion of Chaetoceros vegetative valves compared to resting spores indicates that the marine environment is highly productive, and nutrients generally are not limiting. Epiphytic diatoms, dominated by Cocconeis spp., are observed throughout JKC36, suggesting transport of algal detritus from shallower regions to the benthos. Three foraminiferal assemblages (FAs): Miliammina spp., Globocassidulina spp., and Paratrochammina bartami/Paratrochammina lepida/Portatrochammina antarctica characterize the benthic foraminiferal fauna and reflect affinities with water masses circulating across the Perseverance Drift and tolerance to corrosive bottom waters. The interval 3400–1800 yr BP is marked by high abundances of Globocassidulina spp., indicating incursions of Weddell Sea Transitional Water over the drift site. This interval implies a period of “freshening” of the water column, coinciding with an open-marine or seasonally open-marine environment during the middle-to-late Holocene Climatic Optimum. The interval 1800 yr BP to the present displays characteristics of slightly colder conditions, as indicated by the absence of the calcareous Globocassidulina spp. FA, and the pronounced presence of agglutinated P. bartami/P. lepida/P. antarctica FA, along with other agglutinated species that are indicative of the presence of sea ice. Therefore, this interval is interpreted to represent the onset of Neoglaciation at the northeastern tip of the Antarctic Peninsula. The consistent presence of Miliammina spp. FA corroborates that the sedimentary record represents a productive, open-marine setting with seasonally variable sea ice extent. The Drift is a unique geologic archive that provides an excellent target for future coring based on the preservation of abundant carbonate material for radiocarbon dating and the potential to develop a multi-proxy data set that could offer a robust understanding of the Holocene depositional and paleoclimatic conditions of the northwestern Weddell Sea.

Variations to calcareous nannofossil CaCO3 content during the middle Eocene C21r-H6 hyperthermal event (~ 47.4 Ma) in the Gorrondatxe section (Bay of Biscay, western Pyrenees)

The carbonate content of calcareous nannofossils is dependent on seawater composition. One of the factors that affect seawater chemistry and consequently the degree of calcification in coccolithophores is temperature, as seen in present day warming oceans. The depth at which carbonates are dissolved (Calcite Compensation depth, CCD) can rise due to an increase in HCO3– and decrease in pH, leading to a major dissolution on the seabed and burndown. Similar processes have also been deduced for Eocene hyperthermal events, such as the PETM and ETM2. This study reports changes in coccolith carbonate mass from a hemipelagic setting (Gorrondatxe, at 1500m paleodepth) during the core of a minor Eocene hyperthermal event, namely the C21r-H6 event (47.44–47.32Ma). Image analysis techniques were used to determine differences in the carbonate mass of selected calcareous nannofossil taxa, revealing species-specific patterns. The CaCO3 mass of Chiasmolithus solitus decreased by 50% over the course of the C21r-H6 event, and many specimens also lost their crossed central bars, an additional indication of mass loss; Reticulofenestra sp. (3–5μm) showed a similar trend, but the percentage of mass lost was lower; Toweius pertusus, interpreted as being reworked, mirrored the behaviour of Chiasmolithus solitus, suggesting that the CaCO3 mass loss may have occurred on the seabed, rather than in the water column. In general, it can be concluded that the lysocline rose to 1500m paleobathymetry in the Bay of Biscay during the C21r-H6 event. Formation of corrosive bottom water in the North Atlantic Ocean is regarded as being responsible for the rise in the lysocline.

Size-dependent response of foraminiferal calcification to seawater carbonate chemistry

Abstract. The response of the marine carbon cycle to changes in atmospheric CO2 concentrations will be determined, in part, by the relative response of calcifying and non-calcifying organisms to global change. Planktonic foraminifera are responsible for a quarter or more of global carbonate production, therefore understanding the sensitivity of calcification in these organisms to environmental change is critical. Despite this, there remains little consensus as to whether, or to what extent, chemical and physical factors affect foraminiferal calcification. To address this, we directly test the effect of multiple controls on calcification in culture experiments and core-top measurements of Globigerinoides ruber. We find that two factors, body size and the carbonate system, strongly influence calcification intensity in life, but that exposure to corrosive bottom waters can overprint this signal post mortem. Using a simple model for the addition of calcite through ontogeny, we show that variable body size between and within datasets could complicate studies that examine environmental controls on foraminiferal shell weight. In addition, we suggest that size could ultimately play a role in determining whether calcification will increase or decrease with acidification. Our models highlight that knowledge of the specific morphological and physiological mechanisms driving ontogenetic change in calcification in different species will be critical in predicting the response of foraminiferal calcification to future change in atmospheric pCO2.

Breathing more deeply: Deep ocean carbon storage during the mid-Pleistocene climate transition

Abstract The ∼100 k.y. cyclicity of the late Pleistocene ice ages started during the mid-Pleistocene transition (MPT), as ice sheets became larger and persisted for longer. The climate system feedbacks responsible for introducing this nonlinear ice sheet response to orbital variations in insolation remain uncertain. Here we present benthic foraminiferal stable isotope (δ18O, δ13C) and trace metal records (Cd/Ca, B/Ca, U/Ca) from Deep Sea Drilling Project Site 607 in the North Atlantic. During the onset of the MPT, glacial-interglacial changes in δ13C values are associated with changes in nutrient content and carbonate saturation state, consistent with a change in water mass at our site from a nutrient-poor northern source during interglacial intervals to a nutrient-rich, corrosive southern source during glacial intervals. The respired carbon content of glacial Atlantic deep water increased across the MPT. Increased dominance of corrosive bottom waters during glacial intervals would have raised mean ocean alkalinity and lowered atmospheric pCO2. The amplitude of glacial-interglacial changes in δ13C increased across the MPT, but this was not mirrored by changes in nutrient content. We interpret this in terms of air-sea CO2 exchange effects, which changed the δ13C signature of dissolved inorganic carbon in the deep water mass source regions. Increased sea ice cover or ocean stratification during glacial times may have reduced CO2 outgassing in the Southern Ocean, providing an additional mechanism for reducing glacial atmospheric pCO2. Conversely, following the establishment of the ∼100 k.y. glacial cycles, δ13C of interglacial northern-sourced waters increased, perhaps reflecting reduced invasion of CO2 into the North Atlantic following the MPT.

Foraminiferal proxy response to ocean temperature variability and acidification over the last 150 years in the Santa Barbara Basin (California)

The last 150 years on the California margin have been marked by high variability in precipitation, temperature, sea surface temperature, and ocean chemistry, in response to both natural and anthropogenic changes. Deciphering climate history from ocean sediments is dependent on our understanding of the various influences on marine proxies. Sediment from Santa Barbara Basin (SBB), California (box core SPR0901-10BC) provides an opportunity to compare multiple marine proxies with instrumental records of climate of the past 150 years and extend them beyond the historical record. Here we use three planktonic foraminiferal proxies, δ18Ocalcite, Mg/Ca, and size-normalized shell weight, to reconstruct the surface temperature and δ18Owater in SBB California at near-annual resolution from AD 1850–2000. Results indicate that Globigerina bulloides Mg/Ca reflects the instrumental temperature trends over the last 150 years, with average Mg/Ca temperatures approximately 2 °C lower than instrumental values, due to the slightly deeper habitat preference of G. bulloides. Similarly, temperature is the dominant control on G. bulloides δ18Ocalcite, and both Mg/Ca and δ18Ocalcite reflect variability consistent with changes in the intensification of ENSO over the past 150 years.Shell weights of the planktic foraminifer G. bulloides are inversely correlated with historical sea surface temperatures over the last 150 years in SBB sediments. Greater shell weight associated with cooler surface ocean temperatures does not support the anticipated relationship between coastal upwelling of cool, low pH water and shell thinning due to CO32− undersaturation. However, in contrast to the previous 130 years, G. bulloides shells deposited on the sea floor since the late 1970s exhibit a distinctive smooth shell structure indicative of partial dissolution. We hypothesize that a mid-1970s shift in intermediate water flow from predominantly equatorward to anomalously poleward led to the influx of relatively corrosive bottom water into SBB with subsequent carbonate dissolution at the sediment–water interface.

Microfossil evidence for trophic changes during the Eocene–Oligocene transition in the South Atlantic (ODP Site 1263, Walvis Ridge)

Abstract. The biotic response of calcareous nannoplankton to environmental and climatic changes during the Eocene–Oligocene transition was investigated at a high resolution at Ocean Drilling Program (ODP) Site 1263 (Walvis Ridge, southeast Atlantic Ocean) and compared with a lower-resolution benthic foraminiferal record. During this time interval, global climate, which had been warm under high levels of atmospheric CO2 (pCO2) during the Eocene, transitioned into the cooler climate of the Oligocene, at overall lower pCO2. At Site 1263, the absolute nannofossil abundance (coccoliths per gram of sediment; N g−1) and the mean coccolith size decreased distinctly after the E–O boundary (EOB; 33.89 Ma), mainly due to a sharp decline in abundance of large-sized Reticulofenestra and Dictyococcites, occurring within a time span of ~ 47 kyr. Carbonate dissolution did not vary much across the EOB; thus, the decrease in abundance and size of nannofossils may reflect an overall decrease in their export production, which could have led to variations in the food availability for benthic foraminifers. The benthic foraminiferal assemblage data are consistent with a global decline in abundance of rectilinear species with complex apertures in the latest Eocene (~ 34.5 Ma), potentially reflecting changes in the food source, i.e., phytoplankton. This was followed by a transient increased abundance of species indicative of seasonal delivery of food to the sea floor (Epistominella spp.; ~ 33.9–33.4 Ma), with a short peak in overall food delivery at the EOB (buliminid taxa; ~ 33.8 Ma). Increased abundance of Nuttallides umbonifera (at ~ 33.3 Ma) indicates the presence of more corrosive bottom waters and possibly the combined arrival of less food at the sea floor after the second step of cooling (Step 2). The most important changes in the calcareous nannofossil and benthic communities occurred ~ 120 kyr after the EOB. There was no major change in nannofossil abundance or assemblage composition at Site 1263 after Step 2 although benthic foraminifera indicate more corrosive bottom waters during this time. During the onset of latest-Eocene–earliest-Oligocene climate change, marine phytoplankton thus showed high sensitivity to fast-changing conditions as well as to a possibly enhanced, pulsed nutrient supply and to the crossing of a climatic threshold (e.g., pCO2 decline, high-latitude cooling and changes in ocean circulation).

Corrosive circulation

An ancient carbon release resulted in widespread dissolution of carbonates at the sea floor. Numerical simulations suggest that the pattern of dissolution can be explained by a top-down invasion of corrosive bottom waters from the North Atlantic.

Sudden spreading of corrosive bottom water during the Palaeocene–Eocene Thermal Maximum

The Palaeocene–Eocene Thermal Maximum was associated with warming and seafloor carbonate dissolution. Numerical simulations suggest that the spread of deep corrosive water from the North Atlantic can explain the observed dissolution patterns.

Deep-sea benthic foraminiferal turnover during the early–middle Eocene transition at Walvis Ridge (SE Atlantic)

The early–middle Eocene transition (EMET, ~51–47Ma) was a pivotal time in Cenozoic global climate development, encompassing the end of the Early Eocene Climate Optimum (EECO) and the start of high-latitude cooling, culminating in the early Oligocene Glacial Maximum (34–33Ma). We studied benthic foraminiferal assemblages and stable isotopes at upper abyssal Site 1263 (paleodepth ~1800m) and lower abyssal Site 1267 (paleodepth ~3500m) on Walvis Ridge (SE Atlantic) across the EMET. Benthic δ18O values at both sites rose from ~49.7Ma on, becoming slightly higher at the shallower site than at the deeper site from ~44Ma on. Benthic δ13C values show a broad maximum at ~49–48Ma. Benthic foraminiferal assemblages at both sites are dominated by calcareous taxa, particularly Nuttallides truempyi, indicative of oligotrophic conditions. Large species of Bulimina, indicative of more food-rich conditions, were less rare at the shallower site, where benthic foraminiferal accumulation rates were generally higher. Specimens intermediate in morphology between N. truempyi (a cosmopolitan warm-water species which died out in the latest Eocene) and its extant, also cosmopolitan, cold-water descendant Nuttallides umbonifera (indicative of corrosive bottom waters), are present, as well as various Epistominella species, indicative of seasonal delivery of phytodetritus to the sea floor, and globally more common after the Eocene–Oligocene transition. These assemblages indicate that deep water environments started a gradual evolution from conditions typical for the warm early Eocene greenhouse to those typical for icehouse conditions by the end of the early Eocene. Characteristic early Eocene species such as Aragonia aragonensis and Quadrimorphina profunda persisted at least until 48Ma (the end of the studied interval) at the deeper site, whereas at the shallower site, they had their last appearances at ~49.7Ma, the time of increase in δ18O values, i.e., high latitude and deep-water cooling. The benthic foraminiferal transition towards assemblages typical for a glaciated world thus started close to the end of the early Eocene in the SE Atlantic, with the middle Eocene already representing a transitional world. The faunal changes in the SE Atlantic began earlier at upper abyssal than at lower abyssal depths. This suggests that a more stratified ocean started to develop in the SE Atlantic at the end of the EECO, possibly due to changes in source regions of intermediate water masses due to polar cooling and/or circulation changes around the Antarctic continent.

Palaeoceanographic development in Storfjorden, Svalbard, during the deglaciation and Holocene: evidence from benthic foraminiferal records

Brines can have a profound influence on the relative abundance of calcareous and agglutinated foraminiferal faunas. Here we investigated the distribution of benthic foraminiferal species in four cores from a brine‐enriched environment in Storfjorden, Svalbard. Stratigraphically, the cores comprise the last 15 000 years. The purpose of the study was to reconstruct changes in the palaeoecology and palaeoceanography of Storfjorden in relation to past climate changes, and to identify potential indicator species for brine‐affected environments. The benthic foraminifera in Storfjorden all have widespread occurrences in the Arctic realm. Calcareous species dominated Storfjorden during the deglaciation and early Holocene until c. 8200 a BP. However, agglutinated species increased in abundance whenever conditions became colder with more sea ice and stronger brine formation, such as during the Older Dryas, the Intra‐Allerød Cold Period and the Younger Dryas. Following a moderately cold period with numerous agglutinated foraminifera from c. 8200–4000 a BP, conditions became more changeable from c. 4000 a BP with repeated shifts between warmer periods dominated by calcareous species and colder periods dominated by agglutinated species. The warmer periods show a stronger influence of Atlantic Water, with reduced brine formation and less corrosive conditions at the sea bottom. Conversely, the colder periods show a stronger influence of Arctic water, with higher brine production and more corrosive bottom water. The distribution patterns of the calcareous species are basically the same whether calculated relative to the total fauna (including agglutinated specimens) or relative to calcareous specimens alone. Moreover, the patterns are similar to the patterns found elsewhere along western Svalbard in areas without the influence of brines. No particular species appear to be specifically linked to brine formation. However, the most persistent agglutinated species R. scorpiurus and A. glomerata are also the species most tolerant of the acidic bottom water that normally is associated with brine formation.

Cenozoic bathyal and abyssal ostracods beneath the oligotrophic South Pacific Gyre (IODP Expedition 329 Sites U1367, U1368 and U1370)

This study reports on Paleocene through Pleistocene deep sea ostracod assemblages from carbonate sequences recovered at Integrated Ocean Drilling Program (IODP) Sites U1367, U1368, and U1370 during Expedition 329 to the South Pacific Gyre (SPG). The SPG has the lowest surface primary productivity in the world oceans, and the sedimentary system beneath it has the lowest rates of organic matter flux to the seafloor in the ocean. Results show that this oligotrophic oceanic environment supported abundant and diverse ostracod faunas during the Oligocene, and the middle Miocene suggesting that ostracods are capable of exploiting low levels of food availability in the deep ocean. The SPG records revealed ostracod assemblages following major global climatic transitions: the Cretaceous/Paleocene (~65.5Ma) and the Eocene/Oligocene (~33.5Ma) transitions, and the middle Miocene climatic optimum (~14Ma). The Danian ostracod assemblage is scarce and characterized by Paraphysocythere spp. and Anebocythereis hostizea. High faunal turnover by the early Oligocene gives rise to an assemblage predominantly characterized by typical lower bathyal and abyssal genera Krithe, Poseidonamicus, Argilloecia, and Cytheropteron that are indicative of cold, moderately oxygenated conditions and signal the cooling of the deep ocean after the E/O transition. An increase in abundance of robust epifaunal taxa Bradleya, Henryhowella, Dutoitella, Pennyella, and Rugocythereis, and a pronounced decrease Argilloecia and Cytheropteron characterize the middle Miocene and Pleistocene assemblages, and indicate the presence of more corrosive bottom waters and enhanced oligotrophic conditions in the SPG. However, low variability in the ostracod assemblage taxonomic composition and stratigraphic distribution at Sites U1367 and U1368 suggest that mostly stable paleoceanographic conditions existed in the South Pacific Gyre during the early to the late Oligocene and from the middle to late Miocene (U1368). Declines in ostracod abundances toward the top of the sedimentary sequences at these sites are synchronous with the late Oligocene warming and the late Miocene carbonate crash in the equatorial Pacific, but they seem to also overlap the time when the site locations subsided below the calcium carbonate compensation depth (CCD) as they moved away from the East Pacific Rise.

Abyssal ostracods from the South and Equatorial Atlantic Ocean: Biological and paleoceanographic implications

We report the distribution of ostracods from ∼5000 m depth from the Southeast and Equatorial Atlantic Ocean recovered from the uppermost 10 cm of minimally disturbed sediments taken by multiple-corer during the R/V Meteor DIVA2 expedition M63.2. Five cores yielded the following major deep-sea genera: Krithe, Henryhowella, Poseidonamicus, Legitimocythere, Pseudobosquetina, and Pennyella. All genera are widely distributed in abyssal depths in the world's oceans and common in Cenozoic deep-sea sediments. The total number of ostracod specimens is higher and ostracod shell preservation is better near the sediment–water interface, especially at the 0–1 cm core depths. Core slices from ∼5 to 10 cm were barren or yielded a few poorly preserved specimens. The DIVA2 cores show that deep-sea ostracod species inhabit corrosive bottom water near the carbonate compensation depth (CCD) even though their calcareous valves are rarely preserved as fossils in sediment cores due to postmortem dissolution. Their occurrence at great water depths may partially explain the well-known global distributions of major deep-sea taxa in the world's oceans, although further expeditions using minimal-disturbance sampling devices are needed to fill geographic gaps.

Middle–late Miocene benthic foraminifera in a western equatorial Indian Ocean depth transect: Paleoceanographic implications

Middle through upper Miocene (17–5 Ma) benthic foraminiferal faunas (> 63 μm) from a depth transect in the western equatorial Indian Ocean (DSDP Site 237, ODP Sites 707, 709 and 710; water depth 1500–3800 m) underwent significant faunal changes at ∼ 14–13 Ma and 11–8 Ma. These faunal changes are not easily interpreted, because of the complex factors controlling benthic foraminiferal distribution and abundance. At ∼ 14–13 Ma, the relative abundance of ‘high-productivity’ taxa ( Bolivina, Bulimina, Melonis and Uvigerina species), although highly variable, generally increased at the two shallower sites (237 and 707), as did the percentage of Epistominella exigua (indicative of seasonal productivity) and Nuttallides umbonifera (indicative of corrosive bottom waters) at the two deeper sites (709 and 710); the latter species had a peak in abundance at the deepest site (710) between 11 and 9 Ma. Benthic foraminiferal accumulation rates (BFARs) increased strongly at the shallower sites starting at 11 Ma, peaking between 9 and 8 Ma, and increased weakly at the deeper two sites starting at 11 Ma. Elongate, cylindrical taxa decreased in abundance between 12 and 11 Ma at all sites. The abundance of planktic foraminiferal fragments was, as expected, overall higher at the deeper sites, with some high values after 12–13 Ma at the shallower sites. The faunal changes suggest that overall the food supply to the sea floor increased, but also became more pulsed or seasonal, peaking at 9–8 Ma, while deep and intermediate waters may have become more ventilated. The first phase of faunal change (14–13 Ma) was coeval with the global increase in benthic foraminiferal oxygen isotope values, worldwide turnover in benthic foraminiferal faunas, and possibly increased production of Antarctic Bottom Water (AABW) and Northern Component Water (NCW). The overall faunal turnover may reflect the composite faunal response to increased food input resulting from heightened productivity and increased bottom water ventilation associated with a combination of monsoonal intensification, global cooling and changes in ocean circulation, but there is no unequivocal evidence for the presence of Tethyan Outflow Water (TOW) over the studied time and depth range, in contrast to earlier publications.

PRESERVATION AND COLOR DIFFERENCES IN NUTTALLIDES UMBONIFERA

Abundant Nuttallides umbonifera (Cushman) in Holocene deep sea sediments of the western North Atlantic make them of great value for reconstructing deep sea paleo-environments. This species is particularly abundant in the high-deposition-rate sediments of the Laurentian Fan between Nova Scotia and Newfoundland, where variations in test color came to our attention. Specimens ranged from a white, chalky appearance to a golden, translucent brown. Because we have conducted stable isotope studies using this species, we reasoned it is important to know the basis for the different colors. Living N. umbonifera are brown due to an internal, pigmented, chitinous layer of the test. In the natural environment, tests could be etched by corrosive bottom waters, or by corrosive pore waters on the millimeter to centimeter scale. By lightly subjecting brown specimens to dilute acid, we are able to etch and frost them until their tests appeared to lose their brown color and turn white. By measuring δ 13 C and δ 18 O on large individuals from the same sample, we show that, on average, there is no significant isotopic difference between the brown and white specimens. Thus, we rule out changing bottom water properties as the source of dissolution. Instead, it is likely that bioturbation creates local pockets of reducing conditions which cause etching of some (but not all) specimens in a contemporaneous population.