Variability in tropical thermocline nutrient chemistry on the glacial/interglacial timescale

Abstract

Abstract The organic carbon to calcite rain ratio in the tropical ocean may play a role in regulating global climate through its influence on CO 2 draw down in the upper ocean. The rain ratio may be altered by changes in the nutrient chemistry of the thermocline. Such changes in the tropical Pacific are revealed by the carbon isotope values of thermocline-dwelling planktonic foraminifera ( Neogloboquadrina dutertrei ). The tropical Pacific record over the last 30 kyr in the region south, or along, the equator, which is fed by upwelling of deeper equatorial undercurrent water (EUC), shows a carbon isotope signal distinct from any other in global mixed-layer or benthic environments. This unique record is consistent across the Pacific. Unlike carbon isotope data from other basins and oceanic habitats, the tropical Pacific thermocline tracer shows last glacial values more positive than those of post-glacial times. The isotopic shift occurs in the early deglacial (20–17 kyr) and leads to a deglacial carbon isotope minimum whose timing differs from that seen elsewhere. The deglacial record of the Pacific seems best interpreted as the progressive development of the modern, nutrient-rich thermocline under high-nutrient low-chlorophyll (HNLC) conditions. The eastern tropical Atlantic also has a unique glacial to modern carbon isotope record that is essentially opposite to that seen in the Pacific. Strongly negative values in glacial times differ from those of the ‘standard stratified’ ocean, indicating higher nutrient concentrations. A large shift to more positive values occurs rapidly at about 13–14 kyr. This indicates a marked reduction in thermocline nutrients. The implication is that nutrient flow to the tropics from the deep ocean (via sub-Antarctic sources) was stronger for the Atlantic during the glacial, but was weaker in the Pacific. Transformation to modern conditions occurred in two steps, at 17–18 and 13–14 kyr, a timing that matches shifts in atmospheric CO 2 concentrations and which would be compatible with driving those concentrations higher over deglacial time. The mechanism could be an inherent difference in production regimes between the tropical Pacific and Atlantic.