Deep-sea Ostracod Research Articles

Pleistocene Deep Sea ostracods from the Bering Sea (IODP expedition 323)

The study presents the first Pleistocene (0–1.9Ma) record of Deep Sea ostracods from the Bering Sea, derived primarily from Integrated Ocean Drilling Program Expedition 323, Site U1344 (59°3.0′N, 179°12.2′W, 3171m of water depth). Deep Sea ostracod abundances in the Bering Sea sediments are some of the lowest that have been recorded in bathyal and abyssal marine environments (<1 specimen per sediment gram). In comparison, benthic foraminifera are several orders of magnitude more abundant in the same samples. The humble ostracod assemblage at Site U1344 is predominantly composed of deep water species Krithe sawanensis, Fallacihowella sp. A, Cytheropteron spp., Eucytherura sp., Argilloecia toyamaensis, and Bradleya mesembrina. Less abundant taxa include Munseyella melzeri, Munseyella ristveti, Cluthia sp., Robertsonites hanaii, and Microcythere mediostriata. Some of these taxa (e.g. Fallacihowella sp. A, Bradleya mesembrina, Microcythere mediostriata) are reported for the first time in the North Pacific. The predominance of the genera Krithe, Fallacihowella, Cytheropteron and Argilloecia indicates cold, ventilated bottom waters. The deep Bering Sea ostracod assemblage shares many common and closely related species with continental slope faunas from the Gulf of Alaska, the Okhotsk Sea, the Arctic Ocean, and even the subpolar North Atlantic. A few continental shelf ostracods, such as species of Munseyella and Robertsonites, are present at Sites U1344 and U1343, in the northern slope of the Aleutian Basin. The presence of shallow water ostracods at the Bering Sea slope sites is possibly explained by sea ice rafting. Exceptionally low ostracod abundance in the U1344 record did not permit evaluating links between ostracod faunas and paleoceanographic conditions; however, an increase in ostracod occurrences throughout the middle Pleistocene at Site U1344 appears to correlate with general sea ice expansion in the Bering Sea. High primary surface productivity, high food flux and high sedimentation rates are considered to be the main factors diluting or suppressing Deep Sea ostracods in the Bering Sea, suggesting that ostracods may prefer living in more oligotrophic deep water environments with well oxygenated waters and moderate food supply.

Deep-sea ostracode turnovers through the Paleocene–Eocene thermal maximum in DSDP Site 401, Bay of Biscay, North Atlantic

Abstract Previous low resolution studies suggest that ostracodes, in contrast with deep sea foraminifera, largely survived the massive environmental changes of the Paleocene–Eocene thermal maximum (PETM). In a new high-resolution study from the continental slope (~ 1800 m paleodepth) NE Atlantic, we also find extensive survivorship of ostracode faunas, but this is accompanied by a temporary drop in species diversity and ecological diversity during the PETM. There are 12 common ostracode species before the PETM that are reduced to only two species at the same time as the benthic foraminiferal extinction event. All but three species reappear in the later parts of the PETM and statistical analysis suggests that most of the apparent “Lazarus” species might be found with sufficient sampling of PETM faunas. We find no evidence for an excursion fauna of ostracodes as has been detected in calcareous nannofossils, planktic foraminifera, and benthic foraminifera. However, the ostracode assemblages changed from a relatively diverse ecological assemblage before and after the PETM to one dominated by infaunal species typical of low oxygen conditions during the PETM. The absence of major extinction and the temporary nature of species disappearances are comparable to turnovers in shallow marine ostracodes and stands in sharp contrast to the ~ 50% species-level extinction in benthic foraminifers.

A fossil record of developmental events: variation and evolution in epidermal cell divisions in ostracodes

The carapaces of some ostracode taxa bear reticulate skeletal ridges that outline underlying epidermal cells. This anatomy allows one to identify homologous cells across individuals, to infer the modal sequence of cell divisions that occurs over ontogeny, and to identify individuals with variant cell patterns (e.g., additional or missing cell divisions), even in fossils. Here we explore the variational properties and evolutionary history of this developmental system in the deep-sea ostracode genus Poseidonamicus. Using a sample of over 2000 specimens to capture variation in cell division sequence, we show that phenotypic variation in this system is highly structured: some variants, regions of the carapace, and lineages are much more variable than others. Much of the differences in variation among cells can be attributed to the molt stage in which cells take their final form-cell divisions occurring later in ontogeny are more variable than those earlier. Despite ample variation, only two evolutionary changes in the sequence of cell divisions occur over the 40 Myr history of this clade. The evolutionary changes that do occur parallel the two most common intraspecific variants, suggesting that developmental structuring of variation can have long-term evolutionary consequences. Analysis of the most common variant over the last two molt stages suggests that it suffers a fitness disadvantage relative to the modal form. Such normalizing selection may contribute to the evolutionary conservativeness of this developmental system in the Ostracoda.

Eocene-Pliocene deep sea ostracodes from ODP site 744A, Southern Indian Ocean

The Eocene-Pliocene deep sea ostracodes from the ODP site 744A (Kerguelen Plateau) are herein studied under the taxonomic and paleoecologic aspects. 28 species are identified, being the genera Krithe, Cytherella and Dutoitella the most diversified. A faunal threshold was recorded in the Early Oligocene, which is tentatively explained under the knowledge of the paleoceanographical studies carried out not only in the Kerguelen Plateau but also in adjacent areas. The faunal turnover and variations in both richness and abundance possibly reflect the inception of psychrosphere and the influence of hydrological changes in the preservation of carapaces. Moreover, the influence of those changes on carbonate preservation is discussed as the cause of faunal impoverishment in the upper portion of the core.

Deep-sea ostracods from the South Atlantic sector of the Southern Ocean during the last 370,000 years

We report changes of deep-sea ostracod fauna during the last 370,000 yr from the Ocean Drilling Program (ODP) Hole 704A in the South Atlantic sector of the Southern Ocean. The results show that faunal changes are coincident with glacial/interglacial-scale deep-water circulation changes, even though our dataset is relatively small and the waters are barren of ostracods until mid-MIS (Marine Isotope Stage) 5.KritheandPoseidonamicuswere dominant during the Holocene interglacial period and the latter part of MIS 5, when this site was under the influence of North Atlantic Deep Water (NADW). Conversely,HenryhowellaandLegitimocytherewere dominant during glacial periods, when this site was in the path of Circumpolar Deep Water (CPDW). Three new species (Aversovalva brandaoae, Poseidonamicus hisayoae, andKrithe mazziniae) are described herein. This is the first report of Quaternary glacial/interglacial scale deep-sea ostracod faunal changes in the Southern and South Atlantic Oceans, a key region for understanding Quaternary climate and deep-water circulation, although the paucity of Quaternary ostracods in this region necessitates further research.

Glacial–interglacial variability in deep sea ostracod assemblage composition at IODP Site U1314 in the subpolar North Atlantic

We present a detailed study of glacial/interglacial deep sea benthic ostracod assemblage variability at IODP Site U1314 (subpolar North Atlantic) in relation to the history of ice-rafting events and changes in deep ocean circulation over the past 170 ky. Our records of ostracod diversity, abundance and dissolution and sediment properties (IRD and CaCO 3) show an excellent correspondence to high amplitude orbital and millennial variability observed in the climate records (δ 13C and δ 18O) from neighboring deep water sites, suggesting that the benthic meiofauna fluctuates synchronously with the prevailing oceanographic conditions (surface ocean conditions, deep ocean circulation and water temperature and food flux). Krithe (dominant), Argilloecia and Cytheropteron are the most abundant and diverse genera in association with Rockallia enigmatica. Three ostracod assemblages are recognized. The genera Pennyella, Argilloecia, Pelecocythere, Ambocythere, Pseudobosquetina, Bradleya and Nannocythere are associated with interglacials and interstadials, and possibly reflect increased flux of food to the sediments and more vigorous NADW formation. A transitional assemblage composed of species of Cytheropteron, Xestoleberis and Eucythere is restricted to climatic transitions and indicate moderate environmental conditions and seasonal productivity. A glacial/stadial assemblage is characterized by a temporal predominance of either intermediate-depth and shallow water Arctic/subarctic species (belonging to Cytheropteron, Polycope, Pedicythere, Swainocythere, Cluthia, Heterocyprideis, Elofsonella and Finmarchinella) or abyssal North Atlantic ostracods ( Bythocythere, Dutoitella, Bathycythere and Bythocypris). The influx of high latitude taxa can be partially explained by ice-rafting, but may also represent a shift of the location of intermediate and deep water convection to the area south of Iceland. Therefore the combination of species characteristic of different watermasses during glacials may reflect shifts in the influence of high nutrient southern source water (e.g. AABW) vs. low nutrient GNAIW during glacials.

CLIMATIC INFLUENCES ON DEEP‐SEA OSTRACODE (CRUSTACEA) DIVERSITY FOR THE LAST THREE MILLION YEARS

Ostracodes are small, bivalved crustaceans with the finest-scale fossil resolution of any metazoan, rivaled only by the fossil record of the protistan Foraminifera. This article presents a synthesis of the patterns and possible causes of alpha species diversity variation in benthic deep-sea ostracodes at drilling sites in the North Atlantic and Arctic Oceans. Taken together, these sites represent a period of great climatic variability covering the past three million years. Sediment cores taken from the Mid-Atlantic Ridge show a positive correlation between warm temperatures and high species diversity. These Mid-Atlantic Ridge cores, at the same latitude as northern Spain, show the same positive correlation during the last two glacial-interglacial cycles (200-0 ka [thousands of years ago]) as they do during the pre-glacial Pliocene 2.85-2.4 Ma (millions of years ago). This positive correlation is also found in Pliocene cores from the Rockall Plateau, at the same latitude as Ireland. During the last 200 thousand years, however, this correlation is reversed in cores taken from both the Rockall and Iceland Plateaus. The discovery of high diversity during colder periods in recent high-latitude Rockall and Iceland cores seems to be explained by spikes in diversity caused by ice-rafting events, which would not affect the lower-latitude Mid-Atlantic Ridge. The Heinrich ice-rafting events reduce North Atlantic surface temperatures and salinity every approximately 6-12 ka, dramatically decreasing surface productivity. This increase in diversity during Heinrich events may be explained either by a negative correlation between surface productivity and benthic diversity or by increase in diversity caused by moderate disturbance when ice rafted debris fall to the bottom of the ocean.

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.

Distribution and seasonal dynamics of Boroecia maxima (Ostracoda: Halocypridinae) in the Arctic basin and adjacent Atlantic waters

Based on materials obtained during Soviet and Russian expeditions to the Arctic basin and adjacent Atlantic waters during the period from 1929–1993, as well as data from the literature, we examined the distribution range of the common deep-sea ostracod Boroecia maxima. This species occurs throughout the pelagic water column (down to 3000 m depth and below) and prefers low water temperatures (surface Arctic and intermediate water masses). During the year, at all depths, the ratio between age groups of B. maxima remains virtually unchanged. Breeding apparently continues year-round. During the polar night, B. maxima makes no migration into warm Atlantic water, where wintering predatory zooplankton occur in large numbers, and thus avoids grazing pressure.

Climate change, body size evolution, and Cope's Rule in deep-sea ostracodes

Causes of macroevolutionary trends in body size, such as Cope's Rule, the tendency of body size to increase over time, remain poorly understood. We used size measurements from Cenozoic populations of the ostracode genus Poseidonamicus, in conjunction with phylogeny and paleotemperature estimates, to show that climatic cooling leads to significant increases in body size, both overall and within individual lineages. The magnitude of size increase due to Cenozoic cooling is consistent with temperature-size relationships in geographically separated modern populations (Bergmann's Rule). Thus population-level phenotypic evolution in response to climate change can be an important determinant of macroevolutionary trends in body size.

The question of cosmopolitanism in the deep-sea ostracod fauna: the example of the genus Pedicythere

Deep-sea Pedicythere species which had been earlier considered as pan-abyssal in distribution are shown to be composite species including similar, but separate species in the Pacific, Indian and Atlantic oceans. Morphology and distribution of Pedicythere are analysed. A key to well-described Cenozoic species and forms in open nomenclature which are referred to Pedicythere is presented, together with an annotated checklist of species. Five new species of Pedicythere are described from the South China Sea: P. hirundo sp. nov., P. gibbera sp. nov., P. dentata sp. nov., P. arator sp. nov. and P. nivea sp. nov.

Mid-Pliocene deep-sea bottom-water temperatures based on ostracode Mg/Ca ratios

We studied magnesium:calcium (Mg/Ca) ratios in shells of the deep-sea ostracode genus Krithe from a short interval in the middle Pliocene between 3.29 and 2.97 Ma using deep-sea drilling sites in the North and South Atlantic in order to estimate bottom water temperatures (BWT) during a period of climatic warmth. Results from DSDP and ODP Sites 552A, 610A, 607, 658A, 659A, 661A and 704 for the period Ma reveal both depth and latitudinal gradients of mean Mg/Ca values. Shallower sites (552A, 610A and 607) have higher mean Mg/Ca ratios (10.3, 9.7, 10.1 mmol/mol) than deeper sites (661A, 6.3 mmol/mol), and high latitude North Atlantic sites (552A, 610A, 607) have higher Mg/Ca ratios than low latitude (658A: 9.8 mmol/mol, 659A: 7.7 mmol/mol, 661A: 6.3 mmol/mol) and Southern Ocean (704: 8.0 mmol/mol) sites. Converting Mg/Ca ratios into estimated temperatures using the calibration of Dwyer et al. (1995) [Dwyer, G.S., Cronin, T.M., Baker, P.A., Raymo, M.E., Buzas, J.S., Corrège, T., 1995. North Atlantic deepwater temperature change during late Pliocene and late Quaternary climatic cycles. Science 270, 1347–1351] suggests that mean middle Pliocene bottom water temperatures at the study sites in the deep Atlantic were about the same as modern temperatures. However, brief pulses of elevated BWT occurred several times between 3.29 and 2.97 Ma in both the North and South Atlantic Ocean suggesting short-term changes in deep ocean circulation.

Miocene ostracods from the Tremiti Islands and Hyblean Plateau: biostratigraphy and description of new and poorly known species

The main taxonomic and stratigraphical results of a study of ostracods from the middle–late Miocene of the Tremiti Islands and the Hyblean Plateau are presented. The faunas are mostly typical of deep thermospheric or psychrospheric bathyal environments. Excluding a few, mostly juvenile shallow water contaminants, a total of 127 species were recorded. Twenty-four taxa are discussed herein; of these, 13 species ( Cytherella dissimilis nov. sp., Cytherella parvula nov. sp., Cytherella subtilis nov. sp., Argilloecia triangularis nov. sp., Retibythere ( Retibythere)? claudii nov. sp., Retibythere ( Retibythere)? gibba nov. sp., Cytheropteron ( Cytheropteron) joachinoi nov. sp., Cytheropteron ( Aversovalva) consuetum nov. sp., Cytheropteron ( Aversovalva) trimerumense nov. sp., Cytheropteron ( Aversovalva) turgidum nov. sp., Loxoconcha julianii nov. sp., Buntonia whatleyi nov. sp., Pterygocythere nuda nov. sp.) and a subspecies ( Costa tricostata dorsoarcuata nov. subsp.) are described as new, while six are left in open nomenclature. The stratigraphical distribution of ostracods is expressed in terms of planktonic foraminiferal zones. Information obtained from this study enriches biostratigraphical data on Mediterranean Miocene deep-sea ostracods, especially with respect to the Langhian.

The Eocene/Oligocene boundary and the psychrospheric event in the Tethys as recorded by deep-sea ostracods from the Massignano Global Boundary Stratotype Section and Point, Central Italy

Latest Eocene–earliest Oligocene deep water ostracods from the Massignano Global Boundary Stratotype Section and Point (GSSP), Central Italy, were studied from a palaeoecological perspective to investigate the psychrospheric event in the Tethys. Although quantitative analyses were hampered by the low incidence of fauna, the high-resolution sampling used in this study (148 samples distributed over a 23-m-thick succession spanning 3 m.y.) allowed the following inferences: (1) overall features (e.g. abundance, diversity, composition) of the assemblages suggest a middle–lower bathyal environment with no significant palaeobathymetrical changes; (2) today a similar fauna, notably characterised by the common occurrence of the genus Agrenocythere, would likely be encountered in upper psychrospheric conditions (water masses beneath the permanent thermocline with a temperature range of 4–10°C); (3) although no noticeable faunal turnover has been recognised throughout the succession, minor faunal changes seem to occur in the first part of the section (probable appearance of Legitimocythere presequenta) and to a lesser extent also across the Eocene/Oligocene boundary. The presumed appearance of L. presequenta (latest Eocene) correlates quite well in timing and possible significance with the faunal changes observed in other microfossil groups (foraminifers and dinocysts). This event is possibly linked to a cooling pulse, which might have been caused by the arrival, or increasing advection, of cold bottom waters in the study area. Faunal evolution across the Eocene/Oligocene boundary is thought to be consistent with an intensification of the cooling trend and an increase of primary productivity, which fits fairly well with the δ 13C record. Relationships between the subtle ostracod faunal changes observed in the Massignano GSSP and the main Late Eocene–Early Oligocene psychrospheric event can be only speculative. Nevertheless, we found no evidence that precludes the possibility of interpreting the Massignano ostracod pattern as a subdued, local expression of this major event. Although speculative, this is a reasonable hypothesis, because the investigated area is situated in Western Tethys, far from the main circulation of deep ocean waters.

Late Quaternary deep-sea ostracodes in the polar and subpolar North Atlantic: paleoecological and paleoenvironmental implications

Two sediment cores from the northern North Atlantic, one from the Iceland Plateau and one from the Rockall Plateau, were investigated in order to examine the response of deep-sea benthic ostracodes to climate-related environmental changes since marine isotope stage (MIS) 7. Ostracode fauna was divided into three factor assemblages by using Q-mode factor analyses and diversity was calculated using the Shannon–Wiener index. The Iceland Plateau revealed an ‘interglacial assemblage’ dominated by Henryhowella, a transitional assemblage dominated by Eucythere, and a ‘background assemblage’ that consists of the common taxa Krithe and Cytheropteron. The presence of Henryhowella is linked to conditions that prevailed during the peak interglacial periods (MIS 5e and 1), characterized by increased food supply, well-oxygenated bottom water, and lateral advection. The presence of Eucythere, mainly during the interstadial periods, appears to be related to slightly increased food supply, whereas the ‘background assemblage’ is considered to be opportunistic and able to cope with decreased food supply as interpreted for glacial times. On the Rockall Plateau the opportunistic ‘background assemblage’ (consisting mainly of Krithe, Argilloecia, and Cytheropteron) shows no obvious relation to climate changes. The ‘interglacial assemblage’ consists of several taxa dominated by Pelecocythere but, as on the Iceland Plateau, it also contains Henryhowella. The third assemblage is the ‘glacial assemblage’ that consists of a variety of taxa, several of which are known from the modern Arctic Ocean and the Greenland Sea shelf. Thus, this assemblage indicates glacial conditions on the Rockall Plateau that are comparable to those found in the present-day Arctic Ocean. Diversity calculations revealed higher ostracode diversities in glacial than in interglacial episodes in both cores and particularly high diversities during periods of increased input of iceberg-rafted debris (IRD) in the Rockall Plateau core. Both cores reveal lower surface-water productivity during the glacial compared to the interglacial periods and particularly low productivity during the IRD events, as inferred from carbonate contents. We assume, therefore, that ostracode diversity in the study areas is negatively correlated with food flux.

Faunal changes in Cenozoic deep-sea ostracod assemblages from the South Atlantic and the Southern Ocean and their palaeoceanographical implications

The Cenozoic palaeoceanographical history of the South Atlantic and the Southern Ocean is summarised on the basis of changes in the ostracod faunas of DSDP/ODP Sites 329, 513, 689, 699, 1087 and 1088. The results are mainly compared with the global palaeoceanographical history of Benson. The present study failed to find strong evidence for a sudden psychrospheric event (Sites 689 and 699) in the late Eocene, but there is, however, a decrease in abundance and diversity of ostracods from the mid-/late Eocene to the Oligocene at Sites 689 and 699. This suggests a more gradual palaeoceanographical change over a relatively long time interval. Mid-Miocene faunal changes occur at Site 1087 and coincide with the 16–14 Ma event of Benson. However, the faunal changes are, in contrast to Benson, here marked by a decline in abundance, diversity and faunal turnover. The local differences in faunal changes in mid-Miocene deep-sea ostracod faunas may be related to the initiation of NADW and/or a major expansion of the Antarctic ice sheet. We found little or no evidence for the 6.3–4.9 Ma event, which Benson recorded and related to the effects of a series of events such as the Messinian Salinity Crisis and increased glaciation on Antarctica. Similar to Benson's global data, we have also found a major faunal turnover at Sites 1087 and 1088 at 3.5 Ma. This is possibly related to the effects of a closure of the Straits of Panama, increased production of NADW, AAIW and AABW, and/or a major deglaciation of the Antarctica in the mid-Pliocene.

Cenozoic deep-sea ostracods from Maud Rise, Weddell Sea, Antarctica (ODP Site 689): a palaeoceanographical perspective

Cenozoic palaeoceanography of the Maude Rise, Weddell Sea, Antarctica, has been investigated using Palaeocene to Quaternary deep-sea ostracod faunas from 23 samples of ODP Site 689. The abundance of ostracods is high enough only during the Palaeogene (Palaeocene–Oligocene) to allow palaeoceanographical inferences based on changes in diversity, dominance, endemism and faunal turnover (first and last occurrences). The abundance is particularly high throughout the Palaeocene and Eocene, but declines irreversibly near the Eocene/Oligocene boundary. The diversity increases more or less continuously from the Early Palaeocene to the Middle Eocene, and then it generally decreases throughout the remaining part of the Palaeogene (Middle Eocene–Oligocene); an exception is a positive peak in the Shannon–Weaver index in a single sample in the Late Oligocene. No positive peaks in diversity and taxa originations (first occurrences) at c. 40–38 Ma, occurs at Site 689; so the site provides no evidence for the establishment of the psychrosphere at this time. This corroborates similar regional results from an earlier study of benthonic foraminifera. Explanations for this may be related to Late Eocene–Early Oligocene changes in sedimentology and clay-mineralogy (associated with the progressive cooling of the Antarctica) which could have negatively affected abundance and diversity locally at Site 689. Alternatively, by this time, the ostracod fauna could also have been subjected to selective removal (with possible local extinction) of taxa (due to increased ventilation) or to thanatocoenosis dissolution (due to a decrease in temperature and availability of CaCO 3). A further possibility may be related to the fact that Site 689 was at intermediate water depths and may have remained within older water masses near the Eocene/Oligocene boundary. Failing these explanations, the results could indicate that the Late Eocene–Early Oligocene palaeoenvironmental changes in the world oceans were more gradual and occurred over a longer time interval than the global ostracod data show, at least at southern high latitudes.

Palaeoceanographical changes recorded by Cenozoic deep‐sea ostracod assemblages from the South Atlantic and the Southern Ocean (ODP Sites 1087 and 1088)

Cenozoic palaeoceanography of the SE Atlantic and Southern Oceans has been investigated using Late Eocene/Early Oligocene to Quaternary ostracod assemblages from 49 samples of ODP Sites 1087 and 1088. Although the overall abundance of ostracods is relatively low (means of 17 and 49 specimens per sample at Sites 1087 and 1088, respectively) and there is an apparently high level of endemism (ranging from 50% to 80% at Sites 1087 and 1088), three major changes in the faunal assemblages are identified at Site 1087 (denoted A, B and C) and two at Site 1088 (denoted B′ and C′). The assemblage boundaries, detected on the basis of stepwise changes in the abundance, diversity, dominance, endemism, faunal turnover and relative abundance of common taxa, coincide broadly with previously identified, ostracod-based palaeoceanographical ‘events’ discussed by Benson and co-workers over the last two decades. The data do not extend sufficiently far back to record the initiation of Assemblage A, but the faunal change between Assemblages A and B, marked by a decline in abundance, species diversity and faunal turnover, occurs within the Middle Miocene (NN5-6). It coincides with a previously documented palaeoceanographical ‘event’ at 16–14 Ma which, we suggest, may be related to the initiation of North Atlantic Deep Water (NADW) production and/or an expansion of the East Antarctic ice sheet. Assemblage B′ is subdivided into the two Sub-assemblages B′1 and B′2 mainly on the basis of an increase in diversity, a peak in faunal turnover and a drop in the relative abundance of the genus Krithe in early Late Miocene time (NN9, c. 10.5 Ma). The B′1/B′2 Sub-assemblage boundary cannot be related to any previously documented faunal change in deep-sea ostracods. Changes associated with the boundaries between Assemblages B and C, and B′ and C′, which we believe to be synchronous, both include a decrease in diversity and abundance. In addition, two strong turnover peaks occur near the B′/C′ boundary at Site 1088. The B/C and B′/C′ boundaries coincide with a previously documented midPliocene ‘event’ (3.5 Ma) (NN15-16) which may be linked to putative closure of the Straits of Panama and increased production of NADW, the latter in turn leading to increased production of Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW). Alternatively, fiuctuations in size of the Antarctic ice sheet during possible Pliocene warm periods could indirectly be responsible for the observed midPliocene faunal changes.

Orbital and suborbital variability in North Atlantic bottom water temperature obtained from deep-sea ostracod Mg/Ca ratios

Magnesium/calcium (Mg/Ca) ratios were measured in the deep-sea ostracod (Crustacea) genus Krithe from Chain core 82-24-4PC from the western mid-Atlantic Ridge (3427 m) in order to estimate ocean circulation and bottom water temperature (BWT) variability over the past 200,000 years. Mg/Ca ratios have been used as a paleothermometer because the ratios are controlled primarily by ambient water temperatures at the time the organism secretes its adult carapace. Over the past two glacial–interglacial cycles, Mg/Ca values oscillated between about 7 mmol/mol and 12 mmol/mol, equivalent to a BWT range of 0 to >3.5°C. The lowest values were obtained on specimens from glacial marine isotope stages (MISs) 2, 4 and 6; the highest values were obtained from specimens from the early part of the Holocene interglacial (MIS 1), and also from MISs 5 and 7. These trends suggest that BWTs in the North Atlantic Ocean fluctuate over orbital time scales. Suborbital variability in Mg/Ca ratios and BWT was also observed for the past 100,000 years. Ratios rose from ∼8 mmol/mol to ∼10 mmol/mol (implying a BWT increase of ∼1 to 3°C) during 14 Mg/Ca excursions. The highest ratios were found in Krithe dated at approximately 32, 36–38, 43, 48, 73, 85 and 93 ka. Although the age model for the Chain 82-24-4PC and temporal resolution do not allow precise correlation, some of these deep-sea bottom temperature excursions appear to correspond to Heinrich events recorded in other regions of the North Atlantic and perhaps Dansgaard–Oeschger interstadial events recorded in Greenland ice cores. If confirmed, this would support the hypothesis that millennial-scale oscillations of climate in the North Atlantic are capable of affecting global climate via thermohaline circulation changes.

Deep-sea ostracode species diversity: response to late Quaternary climate change

Late Quaternary ostracode assemblages from the North Atlantic Ocean were studied to establish the effect of climatic changes of the past 210,000 yr (marine oxygen isotope stages 7–1) on deep-sea benthic biodiversity and faunal composition. Two-hundred and twenty five samples from the Chain 82-24 Core 4PC (41°43′N, 32°51′W, 3427 m water depth) on the western Mid-Atlantic Ridge revealed high amplitude fluctuations in ostracode abundance and diversity coincident with orbital and suborbital scale climate oscillations measured by several paleoceanographic proxy records. During the past 210,000 yr, ostracode biodiversity as measured by species number ( S) and the Shannon–Weaver index, H( S), oscillated from H( S)=0.4 during glacial periods (marine isotope stages 6, 5d, 5b, 4, and 2) to H( S)=1.1 during interglacial and interstadial periods (stages 7, 5e, 5c, 5a, 3 and 1). A total of 23 diversity peaks could be recognized. Eleven of these signify major periods of high diversity [ H( S)>0.8, S = 10–21] occurring every 15–20 ka. Twelve were minor peaks which may represent millennial-scale diversity oscillations. The composition of ostracode assemblages varies with Krithe-dominated assemblages characterizing glacial intervals, and Argilloecia–Cytheropteron characterizing deglacials, and trachyleberid genera ( Poseidonamicus, Echinocythereis, Henryhowella, Oxycythereis) abundant during interglacials. Diversity and faunal composition changes can be matched to independent deep-sea paleoceanographic tracers such as benthic foraminiferal carbon isotopes, Krithe trace elements (Mg/Ca ratios), and to North Atlantic region climate records such as Greenland ice cores. When interpreted in light of ostracode species' ecology, these faunal and diversity patterns provide evidence that deep-sea benthic ecosystems experience significant reorganization in response to climate changes over orbital to millennial timescales.