Radiocarbon Evidence for the Contribution of the Southern Indian Ocean to the Evolution of Atmospheric CO₂ Over the Last 32,000 Years

Abstract

It is widely assumed that the ventilation of the Southern Ocean played a crucial role in driving glacial‐interglacial atmospheric CO2 levels. So far, however, ventilation records from the Indian sector of the Southern Ocean are widely missing. Here we present reconstructions of water residence times (depicted as ΔΔ14C and Δδ13C) for the last 32,000 years on sediment records from the Kerguelen Plateau and the Conrad Rise (~570‐ to 2,500‐m water depth), along with simulated changes in ocean stratification from a transient climate model experiment. Our data indicate that Circumpolar Deep Waters in the Indian Ocean were part of the glacial carbon pool. At our sites, close to or bathed by upwelling deep waters, we find two pulses of decreasing ΔΔ14C and δ13C values (~21–17 ka; ~15–12 ka). Both transient pulses precede a similar pattern in downstream intermediate waters in the tropical Indian Ocean as well as rising atmospheric CO2 values. These findings suggest that 14C‐depleted, CO2‐rich Circumpolar Deep Water from the Indian Ocean contributed to the rise in atmospheric CO2 during Heinrich Stadial 1 and also the Younger Dryas and that the southern Indian Ocean acted as a gateway for sequestered carbon to the atmosphere and tropical intermediate waters.

Deglacial Ventilation Changes in the Deep Southwest Pacific

“…. A recent low-resolution record from the Indian Sector of the Southern Ocean shows old deep-water radiocarbon during the ACR and young deep-water radiocarbon during the YD (Ronge et al, 2020). These results differ from the deep South Atlantic reconstructions, which show small radiocarbon variability since the ACR (Skinner et al, 2010).…”

Glacial heterogeneity in Southern Ocean carbon storage abated by fast South Indian deglacial carbon release

“…Data of ref. 16 are available at https://doi.org/10.1594/PANGAEA.906365. Model simulation results of ref.…”

“…In addition, sedimentation in these areas can be highly dynamic with common occurrences of highaccumulation drift deposits 14,15 , often preventing robust age models to be developed. In particular, stratigraphic alignments of sedimentary iron concentrations to Antarctic ice-core dust as recently employed for the Southwest Indian Ocean 16 may be problematic as the lithogenic fraction may likely not be solely of aeolian origin 17,18 . Many of these limitations were circumvented through paired uranium series-and 14 C-dated corals in the Drake Passage upwelling hotspot region 8 complemented by deep-ocean 14 C ventilation reconstructions from the South Atlantic 5 .…”

“…The data show a strengthening of upwelling and deep convection in this region from the last ice age until 14.6 kyr before present (BP), which likely contributed to the observed CO 2,atm rise during HS1. However, given the challenges and potential shortcomings mentioned above 16 , robust estimates of the deglacial evolution of ventilation changes in the deep (South) Indian Ocean are yet limited. While intermediate-ocean 14 C-based ventilation reconstructions offshore the Arabian Peninsula hypothesize two distinct upwelling events in the Indian sector of the Southern Ocean 19 , this hypothesis remains untested and ultimately translates into highly uncertain past global carbon budgets associated with the necessity of extrapolations to the deep Indian Ocean 4,10 .…”

“…b Spatial changes in particle transport in percent across the 1000 m-depth surface, with vectors showing the average speed and direction of ocean currents at mid-depth (1-3.5 km) based on the Global Ocean Data Assimilation System (GODAS) database (https://psl.noaa.gov/data/gridded/data.godas.html) representing the Antarctic Circumpolar Current between 40-60°S. Squares indicate reconstructed deep-water 14 C ages in the Southern Ocean during the last glacial maximum (LGM) referenced to preindustrial 10,16 . Star in both panels shows the location of the study core.…”