Log-ratio of silica to aluminium counts (ln(Si/Al)) from ODP site 108-658

Growing evidence suggests that the low atmospheric CO2 concentration of the ice ages resulted from enhanced storage of CO2 in the ocean interior, largely as a result of changes in the Southern Ocean1. Early in the most recent deglaciation, a reduction in North Atlantic overturning circulation seems to have driven CO2 release from the Southern Ocean**2, 3, 4, 5, but the mechanism connecting the North Atlantic and the Southern Ocean remains unclear. Biogenic opal export in the low-latitude ocean relies on silicate from the underlying thermocline, the concentration of which is affected by the circulation of the ocean interior. Here we report a record of biogenic opal export from a coastal upwelling system off the coast of northwest Africa that shows pronounced opal maxima during each glacial termination over the past 550,000 years. These opal peaks are consistent with a strong deglacial reduction in the formation of silicate-poor glacial North Atlantic intermediate water**2 (GNAIW). The loss of GNAIW allowed mixing with underlying silicate-rich deep water to increase the silicate supply to the surface ocean. An increase in westerly-wind-driven upwelling in the Southern Ocean in response to the North Atlantic change has been proposed to drive the deglacial rise in atmospheric CO2 (refs 3, 4). However, such a circulation change would have accelerated the formation of Antarctic intermediate water and sub-Antarctic mode water, which today have as little silicate as North Atlantic Deep Water and would have thus maintained low silicate concentrations in the Atlantic thermocline. The deglacial opal maxima reported here suggest an alternative mechanism for the deglacial CO2 release**5, 6. Just as the reduction in GNAIW led to upward silicate transport, it should also have allowed the downward mixing of warm, low-density surface water to reach into the deep ocean. The resulting decrease in the density of the deep Atlantic relative to the Southern Ocean surface promoted Antarctic overturning, which released CO2 to the atmosphere.

(Table 1) N-isotope record of planktonc foraminifera of sediment core MD02-2550C2

Constraining variations in marine N2-fixation over glacial-interglacial timescales is crucial for determining the role of the marine nitrogen cycle in modifying ocean productivity and climate, yet paleo-records from N2-fixation regions are sparse. Here we present new nitrogen isotope (d15N) records of bulk sediment and foraminifera test-bound (FB) nitrogen extending back to the last ice age from the oligotrophic Gulf of Mexico (GOM). Previous studies indicate a substantial terrestrial input during the last ice age and early deglacial, for which we attempt to correct the bulk sediment d15N using its observed relationship with the C/N ratio. Both corrected bulk and FB-d15N reveal a substantial glacial-to-Holocene decrease of d15N toward Holocene values of around 2.5 per mil, similar to observations from the Caribbean. This d15N change is most likely due to a glacial-to-Holocene increase in regional N2-fixation. A deglacial peak in the FB-d15N of thermocline dwelling foraminifera Orbulina universa probably reflects a whole ocean increase in the d15N of nitrate during deglaciation. The d15N of the surface dwelling foraminifera Globigerinoides ruber and the corrected bulk d15N show little sign of this deglacial peak, both decreasing from last glacial values much earlier than does the d15N of O. universa; this may indicate that G. ruber and bulk N reflect the euphotic zone signal of an early local increase in N2-fixation. Our results add to the evidence that, during the last ice age, the larger iron input from dust did not lead to enhanced N2-fixation in this region. Rather, the glacial-to-Holocene decrease in d15N is best explained by a response of N2-fixation within the Atlantic to the deglacial increase in global ocean denitrification.