Benthic Foraminiferal Shells Research Articles

Foraminiferal shells in sediment traps: Implications of biogenic particle transport in the Kao-ping submarine canyon, Taiwan

Foraminifera collected from sediment traps deployed at two depths in the Kao-ping submarine canyon were analyzed to provide information on biogenic particle transport. The discovery of benthic foraminiferal shells in the collecting cups was unexpected because the sediment traps were deployed at levels of 54 and 104 m above the seafloor of 290 m depth. The presence of shelf-originated benthic foraminifera captured by the sediment traps suggests that the canyon is not only a conduit for delivering terrestrial materials into the ocean but also acts as a passage allowing particles of marine-origin to be transported toward the shore. Furthermore, during the passing of Typhoon Kai-Tak, the cups at both upper and lower levels collected a higher diversity of benthic foraminifera species than at other time intervals, while there was also an increased similarity in the species collected by all cups. Most of the benthic taxa found in collecting cups were also present at as forms living in surface sediments from the shelf and slope as determined by staining, supporting a shore-wards transport particles of marine origin in the submarine canyon. Nevertheless, the stable isotopic compositions of the benthic foraminifera Cibicides wuellerstorfi displayed a similar range of variation as the other planktonic foraminifera ( Globigerina bulloides, Globigerinoides sacculifer and G. ruber), indicating these species dwelled at a relatively shallow depth (<50 m). The similarity of δ 18O− δ 13C compositions for both planktonic and benthic foraminifera collected from the sediment traps and also from the underlying surface sediments implies that most of the foraminiferal shells precipitated their shells locally and were transported by either settling out of the water column from the adjacent shelf or resuspension from the underlying sea floor.

Modern and Holocene hydrographic characteristics of the shallow Kara Sea shelf (Siberia) as reflected by stable isotopes of bivalves and benthic foraminifera

River discharge of Ob and Yenisei to the Kara Sea is highly variable on seasonal and interannual time scales. River water dominates the shallow bottom water near the river mouths, making it warmer and less saline but seasonally and interannually more changeable than bottom water on the deeper shelf. This hydrographic pattern shows up in measurements and modelling, and in stable isotope records (δ18 O, δ13 C) along the growth axis of bivalve shells and in multiple analyses of single benthic foraminiferal shells. Average isotope ratios increase, but sample‐internal variability decreases with water depth and distance from river mouths. However, isotope records of bivalves and foraminifera of a sediment core from a former submarine channel of Yenisei River reveal a different pattern. The retreat of the river mouth from this site due to early Holocene sea level rise led to increasing average isotope values up core, but not to the expected decrease of the in‐sample isotope variability. Southward advection of cold saline water along the palaeo‐river channel probably obscured the hydrographic variability during the early Holocene. Later, when sediment filled the channel, the hydrographic variability at the core location remained low, because the shallowing proceeded synchronously with the retreat of the river mouth.

Ancient Global Warming

PALEOCLIMATE The Paleocene-Eocene thermal maximum occurred at the close of the Paleocene Epoch about 55 million years ago. Within a few thousand years, sea surface temperatures warmed by 4° to 8°C, and deep ocean temperatures increased by 5°C. This episode, which lasted for approximately 210,000 years, induced a host of biotic responses and produced a large perturbation in the marine and terrestrial carbon isotopic records. It is believed that this event was caused by massive dissociation of marine sedimentary methane hydrates, but the sequence of processes that triggered it is still obscured by the difficulty of determining its timing and duration. Thomas et al. present high-resolution stable isotope records based on analyses of single planktonic and benthic foraminiferal shells, demonstrating that the initial carbon isotope excursion was geologically instantaneous and was preceded by a brief period of gradual surface-water warming. Methane-derived carbon was mixed from the surface ocean downward, suggesting that a significant fraction of the initial dissociated methane hydrate reached the atmosphere before oxidation. This supports the idea that the Paleocene-Eocene thermal maximum was triggered by a small surface ocean warming, followed by the injection into the atmosphere of large amounts of methane, a potent greenhouse gas, derived from marine sedimentary sources. — HJS Geology 30 , 1067 (2002).

Warming the fuel for the fire: Evidence for the thermal dissociation of methane hydrate during the Paleocene-Eocene thermal maximum

Dramatic warming and upheaval of the carbon system at the end of the Paleocene Epoch have been linked to massive dissociation of sedimentary methane hydrate. However, testing the Paleocene-Eocene thermal maximum hydrate dissociation hypothesis has been hindered by the inability of available proxy records to resolve the initial sequence of events. The cause of the Paleocene-Eocene thermal maximum carbon isotope excursion remains speculative, primarily due to uncertainties in the timing and duration of the Paleocene-Eocene thermal maximum. We present new high-resolution stable isotope records based on analyses of single planktonic and benthic foraminiferal shells from Ocean Drilling Program Site 690 (Weddell Sea, Southern Ocean), demonstrating that the initial carbon isotope excursion was geologically instantaneous and was preceded by a brief period of gradual surface-water warming. Both of these findings support the thermal dissociation of methane hydrate as the cause of the Paleocene-Eocene thermal maximum carbon isotope excursion. Furthermore, the data reveal that the methane-derived carbon was mixed from the surface ocean downward, suggesting that a significant fraction of the initial dissociated hydrate methane reached the atmosphere prior to oxidation.

Temperature control on the incorporation of magnesium, strontium, fluorine, and cadmium into benthic foraminiferal shells from Little Bahama Bank: Prospects for thermocline paleoceanography

Surface sediments from Little Bahama Bank (LBB ), intersecting the subtropical thermocline, were used to assess the influence of temperature on the incorporation of Mg, Sr, F, and Cd into shells of benthic foraminifera. Samples were obtained from twelve ☐ cores along the southern slope of LBB, covering a temperature range of 18-4.5°C between 301 and 1585 m. We studied the composition of ten calcitic and one aragonitic species, which are often used in paleochemical reconstructions. Mg/Ca ratios decrease with increasing water depth in all benthic species, both with calcitic and aragonitic mineralogy, showing a strong correlation with water temperature. Similar decrease is seen in Sr/Ca but with no correlation with temperature. None of the benthic species studied here exhibits a depth or temperature related change in F/Ca. Similar trends are observed when using an ocean-wide dataset, which includes shallow and deep core tops (300–5000 m). We suggest that temperature is the primary control on the Mg content of benthic foraminifera. Based on inorganic precipitation experiments and thermodynamic considerations, presented here, a 30–40% decrease in the Mg distribution coefficient in calcite may be expected as a result of a temperature change from 25°C to 5°C, which is about half the observed change in LBB. A calibration curve applied to C. pachyderma data from core tops along the slope of Little Bahama Bank suggests that water temperature may be inferred from Mg/Ca ratios with an uncertainty of about ±0.8°C. Therefore, the Mg content of benthic foraminifera may provide a new, independent temperature proxy for studying shallow waters paleoceanography. The linear decrease in Sr/Ca with increasing depth is not correlated with temperature; the trend is constant from the ocean surface down to 5 km, suggesting that pressure related effects on the calcification process are a more likely explanation than post-depositional dissolution. Mg/Ca ratios in aragonitic shells of H. elegans covary with temperature, in accord with recent observations from corals. In contrast, the Sr and F chemistry of H. elegans is very different than that of corals and inorganically precipitated aragonites. The disparities between the elemental composition of biogenic and inorganic phases and the large intergeneric and interspecific differences observed both in planktonic and benthic foraminifera implicate temperature related physiological processes in regulating the coprecipitation of elements in foraminiferal shells. Our work demonstrates that Cd/Ca ratios of shallow calcitic species reflect the vertical distribution of nutrients; no significant influence of temperature on the partitioning of Cd into the shells was found. Our data extend the previous deep water calibration ( Boyle, 1992), thereby allowing for the reconstruction of the nutrient chemistry of shallow thermocline waters.

Evidence of a dissolution effect on benthic foraminiferal shell chemistry: δ13C, Cd/Ca, Ba/Ca, and Sr/Ca results from the Ontong Java Plateau

Core‐top benthic foraminifera (Cibicidoides wuellerstorfi) from a depth transect of Soutar box cores from the Ontong‐Java Plateau (1.6–4.4 km) were analyzed for cadmium, barium, and strontium (Cd/Ca, Ba/Ca, and Sr/Ca) and for their stable isotopic composition (δ13C and δ18O). We also measured bottom water δ13C, Cd, and Ba at these sites. Foraminiferal δ13C values remain roughly constant over the entire depth range while bottom water δ13C values increase slightly, such that the δ13C difference between C. wuellerstorfi and bottom water ranges from about +0.2 ‰ in cores above 2.5 km to about −0.2 ‰ in cores below 4 km. This apparent depth dependence has not been previously reported, but this range in Δδ13C values is comparable to the uncertainty in published δ13C calibration studies. We observe strong decreases in foraminiferal Cd/Ca, Ba/Ca, and Sr/Ca ratios (50, 25, and 15 percent, respectively) at water depths greater than about 2.5 km. These decreases are substantially larger than the corresponding changes in bottom water trace element concentrations, and they are not correlated with variations in pore water Cd and Ba concentrations at these sites. Together, the foraminiferal and bottom water Cd/Ca, Ba/Ca, and Sr/Ca data yield decreases in the apparent distribution coefficients for these metals into calcite with increasing water depth, again a pattern which has not been previously reported. These results when combined with the data from published core‐top calibration studies suggest that a preferential loss of Cd, Ba, and Sr occurs during the dissolution of benthic foraminiferal calcite on the sea floor and raise the possibility of a dissolution‐driven decrease in benthic foraminiferal δ13C values.

Deep-Sea Foraminifera in the South Atlantic Ocean: Ecology and Assemblage Generation

Benthic foraminiferal assemblages of 237 high-quality surface sediment samples from the South Atlantic Ocean, including the Atlantic sector of the circumpolar ocean, were quantitatively analyzed. Comparisons between assemblages of live (stained with Rose Bengal) and dead specimens showed that potential fossil assemblages of dead specimens were markedly impoverished in fragile and non-resistant agglutinated forms compared to the living assemblages. The dead material was grouped into nine principal faunal end-members by multivariate statistics and quantitatively correlated with available environmental variables. This data set is planned to serve as a reference for interpreting benthic foraminifera fluctuations in Pleistocene Southern Atlantic core material and to constrain the reliability of proxies directly derived from benthic foraminiferal shell geochemistry. Multiple regression analysis in concert with the analysis of distinctive and limited distribution areas of specific faunas delineated ecological demands and preferences of well-known cosmopolitan species and the generation of characteristic assemblages. We differentiated between four principal but interdependent groups of environmental agents acting upon the generation and distribution of nine specific potential fossil assemblages: (I1) Lateral advection and bottom water ventilation, (2) primary productivity and organic carbon flux rates, (3) bottom water carbonate corrosiveness and (4) energetic state at the benthic boundary layer.

Uptake of cadmium into calcite shells by planktonic foraminifera

Planktonic foraminifera were cultured in the laboratory and the uptake of Cd into their calcite investigated using Cd and Sr radiotracers. Best estimates of the effective distribution coefficient of Cd relative to Ca in planktonic foraminiferal calcite are ∼2–4 from these experiments and previously determined values of the Sr distribution coefficient. Within the uncertainty of these estimates, planktonic foraminiferal shells reflect seawater Cd/Ca ratios with a proportionality constant similar to that determined for benthic foraminifera. Cd/Ca ratios in planktonic and benthic foraminiferal shells from sediment cores may be useful therefore for reconstructing water-column profiles of Cd/Ca over time.

Cadmium: Chemical tracer of deepwater paleoceanography

The oceanic distribution of cadmium resembles that of phosphorus. Because the cadmium content of foraminiferal shells is governed by the cadmium content of seawater, planktonic and benthic fossil shells can be used to infer nutrient distributions within ancient oceans. Empirical studies demonstrate that cadmium in benthic foraminiferal shells is related to the bottom water composition through a proportionality constant D ≃ 2.9. This constant is the same for each of the species studied: Cibicidoides wuellerstorfi, Cibicidoides kullenbergi, Nuttallides umbonifera, and Uvigerina.spp. Downcore cadmium data from high‐quality Pacific and Atlantic sediment cores suggest that the cadmium inventory of the ocean did not change significantly between the most recent glacial maximum and the present. Hence changes in the cadmium content of fossils at a site directly reflect changes in nutrient distributions due to altered oceanic circulation patterns. Studies of cadmium in Pleistocene sediments show that deep ocean circulation patterns were significantly different during the most recent glacial maximum. In the western North Atlantic Ocean, the nutrient content of waters from 2500 to 3500 m was twice as high during glacials as during interglacial periods, signifying an increase in the proportion of waters of Antarctic origin in this depth interval and a decrease (but not a cessation) in the flux of North Atlantic Deep Waters through this depth range. Below 3500 m, the nutrient increase was greater, indicating a higher proportion of Antarctic Bottom Water. Above 2500 m, the nutrient content was lower during glacial periods than it has been during interglacial times. The Cd content of high‐latitude North Atlantic and southern ocean surface waters does not show a significant glacial/interglacial change, which argues against theories for changes atmospheric carbon dioxide that require changes in high‐latitude nutrient concentrations.