Sea Level Change, Anaerobic Methane Oxidation, and the
Glacial-Interglacial Phosphorus Cycle
Abstract
The oceanic phosphorus cycle describes how phosphorus moves through the
ocean, accumulates with the sediments on the sea floor, and participates
in biogeochemical reactions. We propose a new two-reservoir scenario of
the glacial-interglacial phosphorus cycle. It relies on diagenesis in
methane hydrate-bearing sediments to mobilize sedimentary phosphorus and
transfer it to the ocean during times when falling sea level lowers the
hydrostatic pressure on the sea floor and destabilizes methane hydrate.
Throughout the cycle, primary production assimilates phosphorus and
inorganic carbon into biomass which, upon settling and burial, returns
phosphorus to the sedimentary reservoir. The impact of the two processes
is not balanced: the former increases the oceanic phosphorus inventory
whereas the latter decreases it. Primary production also lowers the
partial pressure of COin the surface ocean, potentially drawing down
COfrom the atmosphere. Concurrent with this slow ‘biological pump’, but
operating in the opposite direction, a ‘physical pump’ brings metabolic
COenriched waters from deep-ocean basins to the upper ocean. The two
pumps compete, but the direction of the COflux at the air-sea interface
depends on the nutrient content of the deep waters. Because of the
transfer of reactive phosphorus to the sediment throughout a glaciation
cycle, low phosphorus/ high COdeep waters reign in the beginning of the
deglaciation, resulting in rapid transfer of COto the atmosphere. The
new scenario provides another element to the suite of processes that may
have contributed to the rapid glacial-interglacial climate transitions
documented in paleo records.