Steven A Hovan

Broad region of no sediment in the southwest Pacific Basin

A broad region, nearly the size of the Mediterranean Sea, exists in the central South Pacific Ocean that is devoid of sediment and has been so since the Late Cretaceous. The requirements for remaining sediment free are very low biological productivity, a shallow calcite compensation depth, essentially no dust input, and no deposition of hydrothermal oxides and hydroxides. One or two of these conditions are common, but nowhere else do all four occur. The combined effect of these sediment-inhibiting factors is a consequence of crustal age, seawater chemistry, and atmospheric, oceanographic, and physiographic isolation. Furthermore, this unique combination of conditions has prevailed for more than 80 million years.

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).

Evidence for orbital forcing of dust accumulation during the early Paleogene greenhouse

The accumulation of wind blown (eolian) dust in deep-sea sediments reflects the aridity/humidity conditions of the continental region supplying the dust, as well as the “gustiness” of the climate system. Detailed studies of Pleistocene glacial-interglacial dust fluxes suggest changes in accumulation rates corresponding to orbital variations in solar insolation (Milankovitch cycles). While the orbital cycles found in sedimentary archives of the Pleistocene are intricately related to glacial growth and decay, similar global orbital signals recognized in deep-sea sediments of early Paleogene age, the last major greenhouse interval ∼65–45 million years ago, could not have been linked to the waxing and waning of large ice sheets. Thus orbital signals recorded in early Paleogene sediments must reflect some other climate response to changes in solar insolation. To explore the potential connection between orbital forcing and the climate processes that control dust accumulation, we generated a high-resolution dust record for ∼58 Myr old sediments from Shatsky Rise (ODP Site 1209, paleolatitude ∼15°N–20°N). The dust accumulation data provide the first evidence of a correlation between dust flux to the deep sea and orbital cyclicity during the early Paleogene, indicating dust supply responded to insolation forcing during the last major interval of greenhouse climate. Furthermore, the relative amplitude of the dust flux response during the early Paleogene greenhouse was comparable to that during icehouse climates. Thus, subtle variations in solar insolation driven by changes in Earths orbit about the Sun may have had a similar impact on climate during intervals of overall warmth as they did during glacial-interglacial states.