The use of sediment color in paleoceanography: Pliocene-Pleistocene sedimentation in the western North Atlantic.

摘要

Reflectance spectra of sediments collected on the western North Atlantic drifts were used in concert with iron chemistry, carbonate, and organic carbon measurements to evaluate the agents responsible for setting the color in sediments. Lightness is controlled by the carbonate content while the hue is primarily due to the presence of hematite and Fe2+/Fe 3+ changes in clay minerals. Carbonate was successfully estimated via multiple linear regression on raw as well as pretreated reflectance spectra for a total of 4141 direct carbonate measurements. Abundance of Upper Carboniferous spores indicates that the hematite is probably derived from the Permo-Carboniferous red beds of the Canadian Maritimes. Hematite is differentially preserved at the various sites due to differences in reductive diagenesis and dilution by other sedimentary components.; Green-purple lamina couplets occur at the transition between carbonate-rich and carbonate-poor sequences. Major and trace elements, bulk X-ray diffraction and rock magnetic measurements as well as mineralogical, structural, and textural characteristics show that such couplets are places of intense suboxic and sulfatic diagenesis that results in mineral synthesis and metal accumulation. These small scale diagenetic phenomena could distort mineralogical, chemical, and paleomagnetic records.; Long-term color-derived parameters (i.e., carbonate and hematite content) and terrigenous fluxes show that in the last 800 kyr sedimentation pattern changes on the Blake-Bahama Outer Ridge were determined by the sediment delivery to the deep basin as well as circulation changes. Sediment delivery increased during glacials (especially during the last 500 kyr and particularly since Stage 6). A fundamental change in the thermohaline circulation occurred at about ∼500 ka corresponding to the end of the Mid Pleistocene Transition period at the onset of the predominant 100 kyr climate cyclicity. Western Boundary Undercurrent probably intensified after that and became more focused at depths below 3000 m during interglacials while it shoaled to below 2000 m during glacials. Changes in hematite content and sedimentation rate on both Blake-Bahama Outer Ridge and Bermuda Rise show a pulse of sediment via the St. Lawrence outlet at the Pliocene-Pleistocene boundary suggesting that a likely change in the hydrography/physiography of the Laurentide Ice Sheet could have been involved in the climatic and ocean circulation changes at that time.