Isotopic composition of the fine fraction of ODP Site 114-704

Ocean Drilling Program Site 704 in the subantarctic South Atlantic was drilled to investigate the response of the Southern Ocean to climatic and Oceanographic developments during the late Neogene. Stable oxygen and carbon isotopes of fine-fraction (<63 µm) carbonate were analyzed to supplement similar analyses of benthic and planktonic foraminifers. The fine fraction is generally composed primarily of coccoliths, and isotopic analyses of the fine fraction were made to complement the foraminiferal analyses. The isotopic curves thus generated suggest paleoceanographic changes not recognizable by the use of benthic and planktonic foraminifers alone. The global Chron 6 carbon isotope shift, found at 253-244 mbsf (6.39-6.0 Ma) at Site 704 in the planktonic and benthic record, is seen in the fine-fraction d13C record as a gradual decrease from 255 mbsf (6.44 Ma) to 210 mbsf (4.24 Ma). At 170 mbsf, mean d18O values of Neogloboquadrina pachyderma increase by 0.6 per mil-0.7 per mil (Hodell and Ciesielski, 1991, doi:10.2973/odp.proc.sr.114.150.1991), reflecting decreased temperature and increased continental ice volume. Accumulation rates increase by 3.3 times above this depth (which corresponds to an age of 2.5 Ma), suggesting increased upwelling and biologic productivity. Carbon isotopic values of fine-fraction carbonate decrease by about 1.5 per mil at 2.6 Ma; however, no change is recorded in the d13C of N. pachyderma. The fine-fraction d13C shift slightly precedes an average l per mil decrease in d13C in benthic foraminifers. The cause of the benthic d13C shift (most likely due to a change in deep water circulation; Hodell and Ciesielski, 1991) is probably not directly related to the fine-fraction shift. The fine-fraction shift is most likely caused by (1) a change in the upwelling to productivity ratio at this site, with increased upwelling bringing lighter carbon to surface waters, more productivity, and higher sedimentation rates and (2) a change in the particle composition of the fine fraction. The increased upwelling is probably due to a northward migration of the Antarctic Polar Front to a position nearer Site 704.

Isotopic composition of strontium in planktonic foraminiferas from sediments of the DSDP Holes 90-588 and 94-607

Measurements of 87Sr/86Sr on samples of planktonic foraminifers were used to reconstruct changes in the Sr isotopic composition of seawater for the past 8 Ma. The late Neogene was marked by a general, but not regular, increase in 87S/86Sr with two breaks in slope at 5.5 and 2.5 Ma. These times mark the beginning of two periods of steep increase in 87Sr/86Sr values, relative to preceding periods characterized by essentially constant values. During the last 2.5 Ma, 87Sr/86Sr values increased at an average rate of 0.000054/Ma. This steep increase suggests that the modem ocean is not in Sr isotopic equilibrium relative to its major input fluxes. A non-equilibrium model for the modern Sr budget suggests that the residence time of Sr is ~2.5 Ma, which is significantly less than previously accepted estimates of 4-5 Ma. Modelling results suggest that the increase in 87Sr/86Sr over the past 8 Ma could have resulted from a 25% increase in the riverine flux of Sr or an increase in the average 87Sr/86Sr of this flux by 0.0006. The dominant cause of increasing 87Sr/86Sr values of seawater during the late Neogene is believed to be increased rates of uplift and chemical weathering of mountainous regions. Calculations suggest that uplift and weathering of the Himalayan-Tibetan region alone can account for the majority of the observed 87Sr/86Sr increase since the early Late Miocene. Exhumation of Precambrian shield areas by continental ice-sheets may have contributed secondarily to accelerated mechanical and chemical weathering of old crustal silicates with high 87Sr/86Sr values. In fact, the upturn in 87Sr/86Sr at 2.5 Ma coincides with increased glacial activity in the Northern Hemisphere. A variety of geochemical (87Sr/86Sr, Ge/Si, d13C, CCD, etc.) and sedimentologic data (accumulation rates) from the marine sedimentary record are compatible with a progressive increase in the chemical weathering rate of continents and dissolved riverine fluxes during the late Cenozoic. We hypothesize that chemical weathering of the continents and dissolved riverine fluxes to the oceans reached a maximum during the late Pleistocene because of repeated glaciations, increased continental exposure by lowered sea level, and increased continental relief resulting from high rates of tectonism.

Stable isotopic record of late Pliocene and Pleistocene foraminifera of ODP Site 114-704

We studied the stable isotopic and carbonate stratigraphy of ODP Hole 704A to reconstruct the paleoceanographic evolution of the eastern subantarctic sector of the South Atlantic Ocean. Site 704 is well positioned with respect to latitude (46°52.8'S, 7°25.3'E) and bathymetry (2532 m) to monitor past migrations in the position of Polar Front Zone (PFZ) and changes in deep-water circulation during the late Pliocene-Pleistocene. Several important changes occurred in proxy paleoceanographic indicators across the Gauss/Matuyama boundary at 2.47 Ma: (1) accumulation rates of biogenic sedimentary components increased by an order of magnitude (Froelich et al., this volume); (2) planktonic d1 8O values increased by an average of 0.5 per mil; (3) the amplitude of the benthic d18O signal increased; (4) the accumulation rate of ice-rafted detritus increased several fold (Warnke and Allen, this volume); and (5) carbon isotopic ratios of benthic foraminifers decreased by 0.5 per mil, as did the d13C of the fine-fraction carbonate by 1.5 per mil (Mead et al., 1991, doi:10.2973/odp.proc.sr.114.152.1991), but no change occurred in planktonic foraminiferal d13C values. Most of these changes are consistent with more frequent expansions and contractions of the PFZ over Site 704 after 2.47 Ma, bringing cold, nutrient-rich waters to 47°S that stimulated both carbonate and siliceous productivity. The synchronous increase in d18O values and ice-rafted detritus accumulation in Hole 704A indicates that the 2.4 Ma paleoceanographic event included ice volume growth on both Antarctica and Northern Hemisphere continents. The decrease in benthic d13C values indicates that the ventilation rate of Southern Ocean deep water decreased and the nutrient content increased during glacial events after 2.5 Ma. At the Gauss/Matuyama boundary, benthic d13C values of the Southern Ocean shifted toward those of the Pacific end member, indicating a decrease in the relative mixing ratio of Northern Component Water and Circumpolar Deep Water. During the early Matuyama (~2.3 to 1.7 Ma), the PFZ generally occupied a southerly position with respect to Site 704 and carbonate productivity prevailed. Exceptions to these general conditions occurred during strong glacial events of the early Matuyama (e.g., isotopic stages 82, 78, 74, and 70), when the PFZ migrated to the north and opal sedimentation predominated at Site 704. At 1.7 Ma, the PFZ migrated toward the equator and occupied a more northerly position for a prolonged interval between ~1.7 and 1.5 Ma. Beginning at ~1.5-1.4 Ma, surface and bottom water parameters (d18O, d13C, %CaCO3, and %opal) in the subantarctic South Atlantic became highly correlated such that glacial events (d18O maxima) corresponded to d13C and carbonate minima and opal maxima. This pattern is typical of the correlation found during the latest Pleistocene in the Southern Ocean (Charles and Fairbanks, in press). This event coincided with increased suppression of Northern Component Water during glacial events after 1.5 Ma (Raymo et al., 1990, doi:10.1016/0012-821X(90)90051-X), which may have influenced the climatology of the Southern Hemisphere by altering the flux of heat and salt to the Southern Ocean).

(Table 1) Date, deployment location, tag longevity, and satellite data of humpback whales (Megaptera novaeangliae) from the Cook Islands

Humpback whales (Megaptera novaeangliae) undertake extensive seasonal migrations from summer feeding areas in high latitudes to winter mating and calving grounds in tropical waters (Clapham and Mead 1999, http://www.jstor.org/stable/3504352). In the Southern Hemisphere, seven populations are recognized by the International Whaling Commission (IWC). In this study, we report the movements of seven whales satellite-tagged in the Cook Islands, including the first documented migration to an antarctic feeding ground. In September 2006 and 2007 we attached Argos satellite-monitored tags to eight humpback whales of various sex and behavioral classes. All whales were tagged in the nearshore waters of Rarotonga (the largest island in the Cooks group).

Strontium isotope ratios of planktonic foraminifera from late Neogene sediment samples

A curve describing the variation of the strontium isotopic composition of seawater for the late Neogene (9 to 2 Ma) was constructed from 87Sr/86Sr analyses of marine carbonate in five Deep Sea Drilling Project (DSDP) sites: 502, 519, 588, 590, and 593. The strontium isotopic composition of the oceans increased between 9 and 2 Ma with several changes in slope. From 9 to 5.5 Ma, 87Sr/86Sr values were nearly constant at ~0.708925. Between 5.5 and 4.5 Ma, 87Sr/86Sr ratios increased monotonically at a rate of not, vert, similar 1 * 10**-4 per million years. The steep slope during this interval provides the potential for high resolution strontium isotope stratigraphy across the Miocene/Pliocene boundary. The rate of change of 87Sr/86Sr decreases to near zero again during the interval 4.5-2.5 Ma, and ratios average 0.709025. The relatively rapid increase of 87Sr/86Sr between 5.5 and 4.5 Ma must be related to changes in the flux or average 87Sr/86Sr ratios of the major inputs of Sr to the oceans. Quantitative modelling of these inputs suggests that the increase was most probably caused by an increase in the dissolved riverine flux of strontium to the oceans, an increase in the average 87Sr/86Sr composition of river water, or some combination of these parameters. Modelling of this period as a transient-state requires a pulse-like increase in the input of 87Sr to the oceans between 5.5 and 4.5 Ma. Alternatively, the 5.5-4.5 Ma period can be modelled as a simple transition from one steady-state to another if the oceanic residence time of strontium was eight times less than the currently accepted value of 4 Ma. During the time interval of steep 87Sr/86Sr increase, other geochemical and sedimentologic changes also occur including an increase in sediment accumulation rates, a drop in the calcite compensation depth (CCD), and a decrease in the delta13C of dissolved bicarbonate (i.e., "carbon shift"). The simplest mechanism to explain 87Sr/86Sr variation and these related geochemical changes is to invoke an increase in the dissolved chemical fluxes carried by rivers to the oceans. This, in turn, implies increased chemical denudation rates of the continents and shelves during the late Neogene. The increase in chemical weathering rates is attributed to increased exposure of the continents by eustatic regression, intensified glacial/interglacial cycles, and accelerated rates of global tectonism beginning at 5.5 Ma during the latest Miocene.

Strontium isotope ratios of middle Eocene to Oligocene sediments from Maud Rise, Antarctica

A 87Sr/86Sr isotope curve of the middle Eocene to Oligocene was produced from analysis of foraminifera in Ocean Drilling Program Hole 689B, Maud Rise, near the coast of Antarctica. Sediments from the hole are well preserved with no evidence of diagenetic alteration. The sequence is nearly complete from 46.3 to 24.8 Ma, with an average sampling interval of 166 kyr. Excellent magnetostratigraphy in Hole 689B allows calibration to the geomagnetic polarity time scale of Cande and Kent (1992). Marine strontium isotopic ratios were nearly stable from 46.3 to 35.5 Ma, averaging near 0.70773, after which they began to increase. A slow increase began after 40.4 Ma, rising at a rate of only about 8*10**-6/m.y. from base values of 0.707707. From 35.5 Ma to 24.8 Ma the average slope increased to 40*10**-6/m.y. The slope remained constant at least until 24.8 Ma, when the record becomes discontinuous owing to unconformities. We evaluate several possible controls on the marine strontium isotope curve that could have led to the observed growth in 87Sr/86Sr ratios near the Eocene/Oligocene boundary. Three mechanisms are considered, including the onset of Antarctic glaciation, increased mountain building in the Himalayan-Tibetan region, and decreased hydrothermal activity. None of the mechanisms alone seems to adequately explain the increased 87Sr/86Sr ratios during the Oligocene. Glaciation as a weathering agent was too episodic and probably began too late to explain the upturn in marine 87Sr/86Sr ratios. There is evidence that uplift in the Himalayan-Tibetan region began in the Miocene, much too late to control Oligocene strontium isotope ratios. Lastly, hydrothermal flux changes since the Eocene were apparently not great enough alone to account for the rise in marine 87Sr/86Sr ratios. We suggest that a combination of causes, such as decreased hydrothermal activity perhaps followed by increased glaciation and mountain building, might best explain the growth of the marine 87Sr/86Sr curve during the Oligocene.