Coarse Fraction Abundances and the Plio-Pleistocene Record of Iceberg-Rafting at IODP Sites 1305 and 1308, North Atlantic Ocean
Sediments from Sites 1305 and 1308 were sampled with an average time spacing of ~3 ky, and analyzed for coarse fraction abundance (CFA; weight % of the 150 µm 2 mm fraction) by sieving. Present data is the total CFA, including both biogenic particles (predominantly foraminifers) and terrigenous grains (interpreted as iceberg-rafted debris, or IRD). IRD grains have a significantly higher weight per grain, however, so IRD variations are interpreted to drive major variations in total CFA.
The Site 1305 record extends to ~1 Ma, and contains six to seven major CFA peaks in the interval 0 700 ka. CFAs do not appear to correlate directly with marine isotope stages (MIS); e.g., both MIS 6 and MIS 11 have relatively low CFAs. However, larger CFA peaks tend to occur near transitions from an interglacial to a glacial. In the interval 700 ka 1.0 Ma, CFAs vary more frequently, recording local response to the mid-Pleistocene transition.
The Site 1308 record extends to ~1.8 Ma. CFAs vary quasi-cyclically at a period of ~100-150 k.y. to ~950 ka, and CFA maxima tend to occur during MIS-defined glacials; however, not every glacial is marked by a CFA maximum, and a few maxima occur within interglacials or near stage transitions. CFA variations are more rapid before ~950 ka.
Future work will define more clearly the roles of global vs. local controls on iceberg-rafting by: extracting the IRD signal from the CFAs; defining iceberg sources from IRD composition; and evaluating the IRD signals within the oxygen isotope stratigraphy developed for each site.