Iron fertilisation and biogeochemical cycles in the sub-Arctic northwest Pacific during the late Pliocene intensification of northern hemisphere glaciation

Increases in the low-field mass-specific magnetic susceptibility (χ), dropstones and the terrigenous sediment component from Ocean Drilling Program (ODP) Site 882 (~ 45°N) have been interpreted to indicate a major onset of ice-rafting to the sub-Arctic northwest Pacific Ocean during marine isotope stage (MIS) G6 (from ~ 2.75 Ma). In contrast, studies of the terrigenous content of sediments cored downwind of ODP Site 882 indicate that dust and disseminated volcanic ash deposition in the sub-Arctic Pacific increased markedly during MIS G6. To investigate the relative contribution of dust, volcanic ash and ice rafting to the Pliocene χ increase, we present new high-resolution environmental magnetic and ice-rafted debris records from ODP Sites 882 and 885. Our results demonstrate that the χ increase at both sites across MIS G6 is predominantly controlled by a previously overlooked mixture of aeolian dust and volcanic ash. Our findings call into question the reliability of χ as a proxy for ice-rafting to the North Pacific. They also highlight a previously undocumented link between iron fertilisation and biogeochemical cycling in the North Pacific at a key stage during intensification of late Pliocene northern hemisphere glaciation.

Research highlights
► Aeolian dust & volcanic glass control magnetics of Pliocene sediments at Site 882.
► Following 2.73 Ma, covariance exists between dust/volcanic glass (Fe) inputs and nutrient-use.
► Elevated surface water Fe increased biological demand for NO3– relative to Si(OH)4.
► This resulted in high NO3– usage and caused the pool of bioavailable Fe and Si(OH)4 to be under-used.