Samantha C. Bova

Glacial-interglacial changes in central tropical Pacific surface seawater property gradients

Much uncertainty exists about the state of the oceanic and atmospheric circulation in the tropical Pacific over the last glacial cycle. Studies have been hampered by the fact that sediment cores suitable for study were concentrated in the western and eastern parts of the tropical Pacific, with little information from the central tropical Pacific. Here we present information from a suite of sediment cores collected from the Line Islands Ridge in the central tropical Pacific, which show sedimentation rates and stratigraphies suitable for paleoceanographic investigations. Based on the radiocarbon and oxygen isotope measurements on the planktonic foraminifera Globigerinoides ruber, we construct preliminary age models for selected cores and show that the gradient in the oxygen isotope ratio of G. ruber between the equator and 8°N is enhanced during glacial stages relative to interglacial stages. This stronger gradient could reflect enhanced equatorial cooling (perhaps reflecting a stronger Walker circulation) or an enhanced salinity gradient (perhaps reflecting increased rainfall in the central tropical Pacific).

Links between eastern equatorial Pacific stratification and atmospheric CO2 rise during the last deglaciation

It is difficult to untangle the mixed influences of high- and low-latitude climate forcing in the eastern equatorial Pacific (EEP). Here we test the hypothesis that the Southern Ocean drove change in the EEP via subsurface intermediate waters during the last deglaciation. We use the δ18O signature of benthic foraminifera to reconstruct water density changes during the last 25 kyr at three intermediate water depths (370 m, 600 m, and 1000 m) in the EEP. Carbonate δ18O records a combined signature of temperature and salinity and is therefore more closely related to density than temperature or salinity alone. We find that benthic foraminiferal δ18O values decreased first in the subsurface, simultaneously with rising temperatures over Antarctica, and propagated up to the surface within ~3 kyr. The early subsurface response initiated a rapid decrease in density stratification over the upper water column as indicated by reduced δ18O gradients between surface and intermediate depths. Stratification of the upper water column remained low through the termination, with stratification minima reached during Heinrich Stadial 1 and the Younger Dryas (YD), synchronous with the two-part deglacial rise in atmospheric CO2. Centennial-scale shifts toward heavier δ18O signatures at 370 and 600 m during the YD indicate short-lived shifts in the Subantarctic Mode Water/Antarctic Intermediate Water boundary to shallower intermediate depths. We suggest that decreased density gradients during the deglaciation accelerated vertical mixing across the EEP, and potentially the entire South Pacific subtropical gyre, which enhanced CO2 delivery from depth to the surface ocean and atmosphere.

Rapid variations in deep ocean temperature detected in the Holocene

The observational record of deep-ocean variability is short, which makes it difficult to attribute the recent rise in deep ocean temperatures to anthropogenic forcing. Here, we test a new proxy – the oxygen isotopic signature of individual benthic foraminifera – to detect rapid (i.e. monthly to decadal) variations in deep ocean temperature and salinity in the sedimentary record. We apply this technique at 1000 m water depth in the Eastern Equatorial Pacific during seven 200-year Holocene intervals. Variability in foraminifer δ18O over the past 200 years is below the detection limit, but δ18O signatures from two mid-Holocene intervals indicate temperature swings >2 °C within 200 years. More vigorous transport between the surface and deep ocean or stronger eddy variability than that observed in the historical record are potential explanations. Distinguishing externally forced climate trends in deep ocean properties from unforced variability should be possible with systematic analysis of suitable deep sea cores.