Michaele Kashgarian

Marine Reservoir Corrections for the Indian Ocean and Southeast Asia

We have measured radiocarbon in prebomb known-age shells and coral from the Indian Ocean and southeast Asia to determine marine reservoir age corrections. Western Indian Ocean results show a strong 14 C depletion due to upwelling in the Arabian Sea, and indicate that this signal is advected over a wide area to the east and south. In contrast, the surface waters of the South China Sea contain relatively high levels of 14 C, due in part to the input of well-equilibrated water masses from the western Pacific. The easternmost regions of the Indian Ocean are also strongly influenced by the flowthrough of Pacific waters north of Australia.

Deglacial changes in ocean circulation from an extended radiocarbon calibration

Temporal variations in the atmospheric concentration of radiocarbon sometimes result in radiocarbon-based age-estimates of biogenic material that do not agree with true calendar age. This problem is particularly severe beyond the limit of the high-resolution radiocarbon calibration based on tree-ring data, which stretches back only to, about 11.8 kyr before present (BP), near the termination of the Younger Dryas cold period. If a wide range of palaeoclimate records are to be exploited for better understanding the rates and patterns of environmental change during the last deglaciation, extending the well-calibrated radiocarbon timescale back further in time is crucial. Several studies attempting such an extension, using uranium/thorium-dated corals and laminae counts in varved sediments, show conflicting results. Here we use radiocarbon data from varved sediments in the Cariaco basin, in the southern Caribbean Sea, to construct an accurate and continuous radiocarbon calibration for the period 9 to 14.5 kyr BP, nearly 3,000 years beyond the tree-ring-based calibration. A simple model compared to the calculated atmospheric radiocarbon concentration and palaeoclimate data from the same sediment core suggests that North Atlantic Deep Water formation shut down during the Younger Dryas period, but was gradually replaced by an alternative mode of convection, possibly via the formation of North Atlantic Intermediate Water.

Climatic and Hydrologic Oscillations in the Owens Lake Basin and Adjacent Sierra Nevada, California

Oxygen isotope and total inorganic carbon values of cored sediments from the Owens Lake basin, California, indicate that Owens Lake overflowed most of the time between 52,500 and 12,500 carbon-14 (14C) years before present (B.P.). Owens Lake desiccated during or after Heinrich event H1 and was hydrologically closed during Heinrich event H2. The magnetic susceptibility and organic carbon content of cored sediments indicate that about 19 Sierra Nevada glaciations occurred between 52,500 and 23,500 14C years B.P. Most of the glacial advances were accompanied by decreases in the amount of discharge reaching Owens Lake. Comparison of the timing of glaciation with the lithic record of North Atlantic core V23-81 indicates that the number of mountain glacial cycles and the number of North Atlantic lithic events were about equal between 39,000 and 23,500 14C years B.P.

Nearly synchronous climate change in the Northern Hemisphere during the last glacial termination

The climate of the North Atlantic region underwent a series of abrupt cold/warm oscillations when the ice sheets of the Northern Hemisphere retreated during the last glacial termination (17.7–11.5kyr ago). Evidence for these oscillations, which are recorded in European terrestrial sediments as the Oldest Dryas/Bølling/Older Dryas/Allerød/Younger Dryas vegetational sequence,, has been found in Greenland ice cores,. The geographical extent of many of these oscillations is not well known,, but the last major cold event (the Younger Dryas) seems to have been global in extent. Here we present evidence of four major oscillations in the hydrological balance of the Owens basin, California, that occurred during the last glacial termination. Dry events in western North America occurred at approximately the same time as cold events recorded in Greenland ice, with transitions between climate regimes in the two regions taking place within a few hundred years of each other. Our observations thus support recent climate simulations which indicate that cooling of the North Atlantic Ocean results in cooling of the North Pacific Ocean which, in turn, leads to a drier climate in western North America.

Carbonate deposition, Pyramid Lake subbasin, Nevada: 2. Lake levels and polar jet stream positions reconstructed from radiocarbon ages and elevations of carbonates (tufas) deposited in the Lahontan basin

Most of the tufas in the Pyramid Lake subbasin were deposited within the last 35,000 yr, including most of the mound tufas that border the existing lake. Many of the older tufas (> 21,000 yr B.P.) contained in the mounds were formed in association with ground-water discharge. The radiocarbon (14C) ages of the older tufas represent maximum estimates of the time of their formation. Lake Lahontan experienced large and abrupt rises in level at ∼22,000, 15,000, and 11,000 yr B.P. and three abrupt recessions in level at ∼16,000, 13,600, and 10,000 yr B.P. The lake-level rises that were initiated at ∼23,500 and 15,500 yr B.P. are believed to indicate the passage of the polar jet stream over the Lahontan basin. During expansion of the Laurentide Ice Sheet, the jet stream moved south across the basin, and during the contraction of the Ice Sheet, the jet stream moved north across the basin. The bulk of the carbonate contained in the mound tufas was deposited during the last major lake cycle (∼23,500–12,000 yr B.P.), indicating that ground- and surface-water discharges increased at ∼23,500 and decreased at ∼12,000 yr B.P. A lake-level oscillation that occurred between 11,000 and 10,000 yr B.P. is represented by a 2-cm thick layer of dense laminated tufa that occurs at and below 1180 m in the low-elevation tufa mounds and at 1205 m in the Winnemucca Lake subbasin.

Uranium-series and radiocarbon geochronology of deep-sea corals: implications for Southern Ocean ventilation rates and the oceanic carbon cycle

We present new uranium-series and radiocarbon measurements for deep-sea corals from the Southern Ocean. These data are used to reconstruct ventilation ages, both at present and at the end of the last glacial period approximately 16 500 years ago. We apply an improved two-component mixing approach to correct uranium-series dates for contaminant thorium and protactinium present in oxide coatings. Calculated seawater radiocarbon values for contemporary samples decrease with depth in the water column and agree with direct seawater radiocarbon measurements for this area. This indicates that deep-sea corals can accurately record seawater radiocarbon distributions. Two of three glacial samples experienced open-system uranium-series systematics, however, a third sample from the Drake Passage yields concordant thorium and protactinium dates as well as seawater values for initial 234U/238U. This coral yields a ventilation age that is approximately 20–40% greater than modern values for its location. This increase is consistent with published deep-sea coral and calibrated planktonic–benthic foraminifera radiocarbon data, suggesting that the glacial oceans as a whole may have been substantially less ventilated, presumably due to decreased formation of North Atlantic Deep Water. An overall decrease in oceanic mixing rates could have contributed to lower dissolved carbon in surface ocean water and lower atmospheric pCO2 during the past glacial period.

Ventilation changes in the northeast Pacific during the last deglaciation

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.

The California Current of the Last Glacial Maximum: Reconstruction at 42°N based on multiple proxies

Multiple paleoceanographic proxies in a zonal transect across the California Current near 42°N record modern and last glacial maximum (LGM) thermal and nutrient gradients. The offshore thermal gradient, derived from foraminiferal species assemblages and oxygen isotope data, was similar at the LGM to that at present (warmer offshore), but average temperatures were 3.3° ±1.5°C colder. Observed gradients require that the sites remained under the southward flow of the California Current, and thus that the polar front remained north of 42°N during the LGM. Carbon isotopic and foraminiferal flux data suggests enhanced nutrients and productivity of foraminfera in the northern California Current up to 650 km offshore. In contrast, marine organic carbon and coastal diatom burial rates decreased during the LGM. These seemingly contradictory results are reconciled by model simulations of the LGM wind- field, which suggest that wind stress curl at 42°N (and thus open-ocean upwelling) increased, while offshore Ekman transport (and thus coastal upwelling) decreased during the last ice age. The ecosystem of the northern California Current during the LGM approximated that of the modern Gulf of Alaska. Cooling and production in this region was thus driven by stronger open-ocean upwelling and/or southward flow of high-latitude water masses, rather than by coastal upwelling.

Gerardia: Bristlecone pine of the deep-sea?

We measured carbon isotope abundances in the layered, proteinaceous skeleton of a zoanthid Gerardia collected from 620 m depth off the Little Bahama Bank (27°N, 79°W). The δ 14C values decreased from −76% in the outer growth edge to an average of −267% in the center of three portions of the skeleton. These δ 14C data suggest an age for this living organism of 1800 ± 300 years. The possibility that the large decrease in δ 14C reflects the gradual input of bomb 14C over the entire growth of the organism is inconsistent with the post-bomb δ 14C values obtained for the most recent growth tips. If the age estimate of two millennia is correct, it may be the longest-lived animal yet observed in the ocean. Gerardia may serve as a long-lived recorder of ocean chemistry, similar to the Bristlecone pine tree that has served as a millennial-timescale recorder for atmospheric 14CO2 (Suess, 1980) and climate. In particular, there is potential for Gerardia to serve as a millennial-scale integrator of upper ocean particle flux, and possibly reveal past changes in the productivity of the surface ocean.

Southwest Subtropical Pacific Surface Water Radiocarbon In A High-Resolution Coral Record

We have generated a high-resolution coral 6,14(: record from the southwest subtropical Pacific spanning the last 50 yr. Prebomb (1950-1956) 6,t4C values average -52%0. Values begin to increase in 1957, reaching a maximum in the early 1970s, about 10 yr after the atmospheric peale There is a consistent 10-15%0 seasonal cycle whose relationship with vertical mixing evol ves as a consequence of the penetration of the bomb transient into subsurface waters. Comparison of this record with that simulated in an ocean general circulation model highlights the difficulty in modeling vertical exchange processes.