Mathieu Richaud

Glacial marine nutrient and carbon redistribution: Evidence from the tropical ocean

The nature and timing of marine nutrient and carbon redistribution through a glacial cycle remains unclear. Understanding transfers to and from the surface and deep ocean reservoirs is important to explaining Pleistocene variation in atmospheric CO2 content. Observations in the modern ocean show that the nutrient supply to the tropical upwelling regions depends on content of deep reservoirs and vertical mixing in the Southern and subantarctic oceans. Previous work in the Pacific demonstrated that nutrient supply to the tropics was reduced during the Glacial, consonant with reduced vertical mixing in the Southern Ocean. We examine the glacial record of the tropical Atlantic with the same methods used in the Pacific (N. dutertrei carbon isotope data combined with export production estimates to evaluate changes in thermocline nutrient content). In contrast to the Pacific, we find evidence for an increase in tropical Atlantic nutrient supply under glacial conditions. The source of nutrients can be traced to subantarctic surface waters and ultimately to an enriched abyssal reservoir. Bathymetrically forced vertical mixing could account for the transfer of nutrients from this reservoir in the S. Atlantic. The enriched reservoir developed in early MIS 4 (75 ka) and persisted until about 14.5 ka (Bolling/Allerod). This timing corresponds to shifts in atmospheric CO2 content from intermediate to minimum (full glacial) levels at 75 ka and from intermediate to Holocene concentrations near 14.5 ka.