Stable oxygen and carbon isotope composition of benthic foraminifera from the western Mediterranean Sea

The influence of microhabitat, organic matter flux, and metabolism on the stable oxygen and carbon isotope composition of live (Rose Bengal stained) and dead (empty tests) deep-sea benthic foraminifera from the Gulf of Lions (western Mediterranean Sea) have been studied. The total range of observed foraminiferal isotope values exceeds 1.0 per mil for d18O and 2.2 per mil for d13C demonstrating a wide range of coexisting disequilibria relative to d18O of equilibrium calcite (d18OEQ) and d13C of bottom water dissolved inorganic carbon (d13CDIC). The mean d18O values reveal strongest disequilibria for the studied epifaunal to shallow infaunal species (Cibicidoides pachydermus, Uvigerina mediterranea, Uvigerina peregrina) while values approach equilibrium in deep infaunal species (Globobulimina affinis, Globobulimina pseudospinescens). The mean d13C values decrease with increasing average living depths of the different species, thus reflecting a dominant microhabitat (pore water) signal. At the axis of the Lacaze-Duthier Canyon a minimum d13CDIC pore water gradient of approximately -2.1 per mil is assessed for the upper 6 cm of the surface sediment. Although live individuals of U. mediterranea were found in different depth intervals their mean d13C values are consistent with calcification at an average living depth around 1 cm. The deep infaunal occurrence of U. mediterranea specimens suggests association with macrofaunal burrows creating a microenvironment with geochemical characteristics similar to the topmost centimeter. This also explains the excellent agreement between stable isotope signals of live and dead individuals. The ontogenetic enrichment in both d18O and d13C values of U. mediterranea suggests a slow-down of metabolic rates during test growth similar to that previously observed in planktic foraminifera. Enhanced organic carbon fluxes and higher proportion of resuspended terrestrial organic material at the canyon axis are reflected by d13C values of U. mediterranea on average 0.58 per mil lower than those from the open slope. These results demonstrate the general applicability of the d13C signal of this species for the reconstruction of past organic matter fluxes in the Mediterranean Sea. Further studies on live specimens are needed for a more quantitative paleoceanographic approach.

Cenomanian-Turonian benthic foraminifera of the Kerguelen Plateau

Recent drilling on the Kerguelen Plateau (Ocean Drilling Program Leg 183) has provided a unique and exciting high latitude record of palaeoceanographic change during the Cenomanian-Turonian in the Southern Ocean. The benthic foraminiferal succession at Site 1138 records the evolution of the Kerguelen Plateau from a subaerially exposed platform in the Cenomanian to a bathyal, pelagic environment in the early Turonian, following a major transgressive pulse and increased thermal subsidence of the Kerguelen Plateau, which led to a sea-level rise of possibly several hundred metres. Diversified benthic foraminiferal assemblages indicate an upper bathyal, mesotrophic setting after the peak of the transgression. The assemblages exhibit strong similarities to temperate, shelf and slope assemblages in the Northern Hemisphere. This bimodal distribution reflects the existence of open oceanic gateways and a dynamic trans-hemispheric global circulation. Equatorial assemblages are characterized by a low-diversity, high carbon flux biofacies. Assemblages from Alaska demonstrate high organic productivity and low oxygen conditions and the prevalence of elevated temperatures on the flooded shelf of the North Slope. Our results show that the distribution of upper bathyal benthic foraminifera was strongly modulated by carbon flux and oxygenation fluctuations, and not by physical migration barriers.

IRD, benthic foraminifera and stable isotopic record of northern Sargasso Sea sediments

New geochemical proxy data from Bermuda Rise piston cores reveal ocean and climate conditions in the northern Sargasso Sea during marine isotope stage 3. Using ?18O on the planktonic foraminifer Globigerinoides ruber, we can correlate explicitly with every stadial/interstadial change in Greenland ice between ~32 and 58 ka. These data suggest repetitive changes of ~4°C in the annual average sea surface temperature (SST), with maximum cooling comparable to or greater than SST during glacial maximum conditions. The extent of SST depression is about the same for typical stadial events and for Heinrich events 4 and 5, which we have identified on the Bermuda Rise by traces of ice rafting. Benthic foraminiferal d13C decreases during every stadial event, consistent with reduced production of the deepest component of North Atlantic Deep Water and shoaling of its interface with Antarctic Bottom Water. This interpretation is supported by benthic Cd/Ca data from the climate cycle associated with interstadial 8.

Benthic Foraminifera in Upper Pleistocene - Holocene bottom sediments from the southeastern Sea of Okhotsk

A comparative analysis of benthic foraminiferal assemblages in the last glacial sediments obtained by gravity cores from the southern Kamchatka slope (Vulk-34-98) and from the eastern slope of the Akademii Nauk Rise in the central Sea of Okhotsk (Vulk-34-90) revealed, along with their undoubted similarity, substantial differences caused by hydrological regime in these areas during the considered period. It is shown that during the last glacial period bottom waters near the northern Kuril Islands were warmer and less aerated than those in the Akademii Nauk Rise area. As is evident from low-amplitude variations in proportions of dominant species, hydrological parameters in the bottom layer of the latter area at that time were relatively more stable than in the former area.

Eocene to Quaternary benthic foraminifers from the Izu-Bonin Arc

The benthic foraminifer fauna at Sumisu Rift Sites 790 and 791 indicates that a deep open-ocean (>2300 m) or a basin with open-ocean access existed between 1.1 and 0.7 Ma at the time of the initiation of rifting. The appearance of a low- to medium-oxygen fauna (1600-2300 m) between 0.7 and 0.5 Ma suggests that the open-ocean access may have been terminated at this time because of the development of volcanoes and rift flank uplifts around the basin. The occurrence of low-oxygen faunas at 0.03 Ma suggests a secondary closing of the basin. The lower bathyal benthic faunas from lower Pliocene sediments of rift margin Site 788 suggest about 0.6-1.6 km of total basement uplift. This uplift may have led to the formation of the major hiatus between 2.3 and <0.3 Ma. The faunal changes of benthic foraminifers at Sites 792 and 793 in the forearc basin document a shallowing water depth from below the carbonate compensation depth (CCD) (about 3.5 km) in the late early Oligocene to the present depths of 1800 and 2975 m, respectively. These data suggest about 1 km of total basement uplift in the inner part of the forearc basin (Site 792) and about 0.6 km total basement subsidence in the central part of the forearc basin (Site 793) since about 31 Ma. The former uplift led to a thinner sediment accumulation (800 m) and the latter subsidence to a thicker sediment accumulation (1400 m) at these sites. Faunal changes of benthic foraminifers observed in Sites 782 and 786 sequences drilled at the outer-arc high document a deepening water depth from 1.3 to 2.1 km in late Eocene to the present depth of about 3 km. These data suggest about 1.1-1.9 and 1.3-2.1 km of total basement subsidence at Sites 786 and 782, respectively. These results indicate total basement uplift in the inner part of the Bonin arc-trench system since late Oligocene and total basement subsidence in the outer part of the system since late Eocene. The last occurrence (LO) of Stilostomella spp. and Pleurostomella spp. and the first occurrence (F0) of Bulimina aculeata d'Orbigny occurred consistently at 0.7 Ma at all three arc proximal sites (790,791, and 792). This fact is taken to suggest a change of water mass, from one originating from the central part of the ocean to that originating from ocean-margin areas at that time.

Live benthic foraminifera in surface sediments of the Indo-Pakistan continental margin

The Indo-Pakistan Continental Margin represents an extreme habitat for benthic foraminifera since (1) high fluxes of organic matter offer a high food supply, (2) an intensified oxygen minimum Zone (OMZ) develops from the base of the euphotic Zone to water depths over 1000 m and (3) the monsoon causes seasonal oscillations within the biogeochemical cycle. At three stations from the uppermost (233 m), the central (658 m) and the deeper part (902 m) of the OMZ, living benthic foraminiferal assemblages were analyzed within the uppermost 10 cm of the sediment column. The ecologic structure of foraminiferal faunas is characterized by high abundances at the sediment surface and a rapid decrease within the uppermost 2 cm of the sediment column. Despite dysoxic to suboxic bottom-water conditions, stained benthic foraminifera occurred in all cores down to the base of the sampled interval. High surface abundances, a high dominance by few endobenthic calcareous taxa and a low diversity, which may result from specific physiological adaptations to almost anoxic conditions and the absence of predators, are recognized in the central part of the OMZ. The upper and lower margins of the OMZ are characterized by higher diversities and lower abundances. The shallowest part of the OMZ is dominated by calcareous foraminifera, whereas agglutinated species are the most common taxa in the deeper part. Comparisons with previous studies show that benthic foraminiferal assemblages, that are influenced by seasonal oscillations controlling food supply and/or the availability of oxygen, show variations in faunal density and species composition. Since there is strong evidence that oxygen is not a limiting factor for some taxa, it seems more likely that the distribution pattern of benthic foraminifera is preferentially controlled by trophic conditions.

Latest Miocene-Pleistocene benthic foraminiferal record of DSDP Hole 23-219

Knowledge of the biology of deep-sea benthic foraminifera was used to interpret the results of multivariate analyses (factor and cluster) on relative abundance data of benthic foraminifera at Deep Sea Drilling Project Site 219 (southeastern Arabian Sea; 1764 m depth) in combination with carbon and oxygen isotope data. Faunal data document major changes in deep-sea ventilation and productivity over the past 5.5 Ma, including the end of the Miocene-Pliocene Indo-Pacific 'biogenic bloom' period at ~4.0 Ma. Interestingly, there is no simple correlation between high productivity and low oxygenation. Productivity fluctuated but became overall more pulsed, whereas overall oxygenation increased. We interpret the records as a combination of local to regional fluctuations in productivity probably caused by changes in monsoonal development, particularly its seasonality, and changes in oxygenation of intermediate depth waters in the Indian Ocean caused by global changes in climate and ocean circulation.