Showing papers by "Richard G. Fairbanks published in 1996"

Neogloboquadrina pachyderma (sinistral coiling) as paleoceanographic tracers in polar oceans: Evidence from northeast water polynya plankton tows, sediment traps, and surface sediments

Abstract: The abundance and chemistry of the planktonic foraminifera Neogloboquadrina pachyderma (sinistral coiling) have long been used as tools for monitoring polar surface ocean changes and for correlating these changes to atmospheric and thermohaline circulation fluctuations. However, due to its remote habitat, very little is known about how modern N. pachyderma (s.) respond to changing environmental conditions in the polar seas. Modern samples of N. pachyderma (s.) from the Northeast Water Polynya provide a means for studying how environmental conditions affect the vertical distribution and chemistry of this species. Highest abundances of N. pachyderma (s.) were associated with the chlorophyll maximum in the surface 20–80 m, where they are exploiting their primary food source. Evidence suggests that the addition of a calcite crust modifies the calcite tests of some N. pachyderma (s.) between 50 and 200 m, increasing shell density and modifying shell chemistry. The shell mass of encrusted forms is 3–4 times greater than the nonencrusted forms between 50 and 200 m. The oxygen isotope composition of N. pachyderma (s.) shells increase by 1.5‰ in response to local water column gradients. The δ13C values of N. pachyderma (s.) are basically invariant with depth in this region, are consistently 1.0‰ depleted in comparison with the δ13C for equilibrium calcite, and remain basically constant during the shell-thickening process. Mass balance calculations suggest that encrustation occurs at all depths, but abundance counts suggest that the process occurs mostly at the depth of the main pycnocline. Sediment fluxes of N. pachyderma (s.) occur during a 2-week bloom event and decrease to almost zero below complete ice cover. The decoupling of the processes controlling abundances and shell chemistry explain the discrepancies between transfer function and isotopically derived paleotemperature estimates of surface conditions, in some oceanic settings. The ability of δ18O to record surface ocean conditions will depend on vertical water column gradients, as evidenced by the differences in core-top calibrations between the North and South Atlantic.

Isotope data from Ice Station Weddell: Implications for deep water formation in the Weddell Sea

Abstract: Helium isotope data (3He/4He ratios and 4He concentrations) and H218O/H216O ratios obtained from stations occupied during the drift of Ice Station Weddell (February to June 1992) are used, together with hydrographic data, to study formation of deep and bottom water in the western Weddell Sea. The data indicate deep and bottom water formation along the entire track of the ice station (71.4 to 65.8°S, ≈53°W). Ice Shelf Water (ISW) seems to contribute significantly to the formation of Weddell Sea Deep Water (WSDW) and Weddell Sea Bottom Water (WSBW) in the southern part of the drift track. Toward the north, the fraction of ISW contained in WSDW/WSBW decreases. This trend is overlaid by high ISW fractions in the deep and bottom waters found in the vicinity of the Larsen Ice Shelf. The fraction of Western Shelf Water (WSW) in WSBW shows the opposite trend, increasing from south to north. The combined fraction of ISW and WSW in waters with potential temperatures below 0°C is about 20%, corresponding to a roughly 200 m thick layer. Overall, WSW seems to contribute approximately 2 to 3 times more water than ISW to the water column below the 0°C isotherm. Using the estimated flow of ISW over the sill north of the Filchner Depression of 1 Sv [Foldvik et al., 1985a] together with the ratio of WSW plus Winter Water (WW) to ISW, we calculate a value of about 5 Sv for the formation rate of WSBW with a potential temperature of −0.7°C. About one third of this flux represents near-surface waters (WSW/WW) which have recently been equilibrated with the atmosphere, whereas pure ISW contributes about 10%.

Ventilation changes in the northeast Pacific during the Last Deglaciation

Abstract: Under present climate conditions, convection at high latitudes of the North Pacific is restricted to shallower depths than in the North Atlantic. To what extent this asymmetry between the two ocean basins was maintained over the past 20 kyr is poorly known because there are few unambiguous proxy records of ventilation from the North Pacific. We present new data for two sediment cores from the California margin at 800 and 1600 m depth to argue that the depth of ventilation shifted repeatedly in the northeast Pacific over the course of deglaciation. The evidence includes benthic foraminiferal Cd/Ca, 18O/16O, and 13C/12C data as well as radiocarbon age differences between benthic and planktonic foraminifera. A number of features in the shallower of the two cores, including an interval of laminated sediments, are consistent with changes in ventilation over the past 20 kyr suggested by alternations between laminated and bioturbated sediments in the Santa Barbara Basin and the Gulf of California [Keigwin and Jones, 1990; Kennett and Ingram, 1995; Behl and Kennett, 1996]. Data from the deeper of the two California margin cores suggest that during times of reduced ventilation at 800 m, ventilation was enhanced at 1600 m depth, and vice versa. This pronounced depth dependence of ventilation needs to be taken into account when exploring potential teleconnections between the North Pacific and the North Atlantic.

Interocean exchange of glacial North Atlantic Intermediate Water: Evidence from subantarctic Cd/Ca and carbon isotope measurements

Abstract: New measurements of Cd/Ca ratios for glacial benthic foraminifera from the continental shelf and slope off Tasmania and South Australia are presented. When combined with glacial carbon isotope measurements [Lynch-Stieglitz et al., 1994], these data from the glacial Subantarctic Zone suggest that a water mass with a high value of the conservative tracer δ13Cas occupied not only the shallow (1–2 km) portions of the Atlantic and Pacific Oceans but was present in the glacial Subantarctic as well. The oceanic Cd concentrations, Cdw, predicted for the glacial Subantarctic are intermediate between those from the glacial Atlantic and the glacial Pacific, supporting the idea that Glacial North Atlantic Intermediate Water may have reached the intermediate depth Pacific via the Antarctic Circumpolar Circulation [Lynch-Stieglitz and Fairbanks, 1994].

A 27,000 year record of Red Sea Outflow: Implication for timing of post‐glacial monsoon intensification

Abstract: We reconstruct here the history of the Red Sea Outflow (RSO) over the past 27,000 years from an AMS 14C-dated high-resolution δ13C record of benthic foraminifera from the inner Gulf of Aden assuming the dominance of circulation over productivity in regulating benthic δ13C. The results reveal that, following a period of suppressed RSO due to shallow sill 24,000–18,000 yr BP, the Red Sea was vigorously flushed for ∼2,000 years before a major monsoon intensification caused the cessation of deep water formation from 15,500 to 7,300 yr BP. It appears that the monsoon intensification did lag behind insolation until 15,500 yr BP. Between 15,500 and the present, however, there was no lag in conflict with the previous reports, implying a negligible dampening effect of continental albedo during this period. However, since our analysis is confined to a single depth horizon and our record is sensitive to sea level, it has some limitations as an indicator of monsoon intensity.