Coarse Fraction Abundances and the Plio-Pleistocene Record of Iceberg-Rafting at IODP Sites 1305 and 1308, North Atlantic Ocean

Plio-Pleistocene orbital- and suborbital-scale climatic variability in the North Atlantic was investigated during IODP Expedition 303, which cored Sites 1305 (southwestern Eirik Drift, 3459 mbsl) and 1308 (reoccupation of DSDP Site 609, central North Atlantic, 3871 mbsl).

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.