Bioproductivity in a warmer world

Problem statement

Pelagic carbonate producers play a key role in the marine carbon cycle by exporting carbon to the deep ocean. Understanding how and why pelagic carbonate production changed in the past in response to environmental forcing, especially during warm periods of the recent Earth history is necessary to constrain its future under projected climate change scenarios. Past changes in carbonate production can be derived from sedimentary records of carbonate accumulation in settings where the effect of post depositional dissolution is negligible. Here we make use of the uniquely resolved and orbitally dated sediment records from Ceara Rise to asses the nature and the origin of variations in pelagic carbonate production in the tropical Atlantic. We generated new high-resolution records of CaCO₃accumulation rate (AR) at ODP Site 927 for the Quaternary interglacials MIS 5, MIS 9, for the Pliocene warm period (PWP) and for the Mid-Miocene climate optimum (MMCO). We observe that pelagic carbonate production at the studied site varied in pace and in phase with local insolation, rather than with global climate (ice volume).Beyond the orbital variations, we observe a secular signal indicating a ~ 50 % decrease in AR from the Miocene towards the Quaternary. This trend does not appear to occur due to orbital variability and cannot be explained by dissolution or age model uncertainty and we conclude that the mean pelagic carbonate production at the studied site differed among the climate states. Given this observation, it is now of interest to interrogate the causes of these differences and reveal its mechanism by evaluating the relative contribution and biomineralisation intensity of the main pelagic calcifiers.

Figure: The comparison between the high-resolution record for the four periods of interest (d and e) and the environmental parameters (a, b and c) is highlighting the good coherence between the Carbonate accumulation rate (AR) (reflecting the pelagic carbonate production) and the local insolation signal. The lack of coherence with the global pCO2 and with the global and local d18Obf (reflecting the ice volume) is underlining a response more local than global, answering at low latitude to the insolation changes more than to the climate signals at high latitude. The observed changes with the d18O are due to the ocean circulation changes when the ice volume is increasing at high latitude. During the Pleistocene, we observe very low values of CaCO3 % (e) and CaCO3 AR (d) during the glacial periods, meaning an important production of North Atlantic Deep Water (NADW), resulting in dissolution in the deep ocean. The observed decrease in CaCO3 % (e) and CaCo3 AR (d) during the two Pleistocene interglacials are due to both the decrease of local insolation and the dissolution due to colder period (and increase of NADW). Looking at the boxplots of the CaCO3 AR of bulk sediment for the four periods (d), we observe a ~ 50 % increase of CaCO3 AR between the Pleistocene and the early Miocene, tracing a pelagic carbonate production in average more important during the Miocene (warmer and more important pCO2) than during the Pleistocene. Concerning the variability, we observe a much more important variability of the pelagic carbonate production signal for the two interglacials of the Pleistocene than for the Miocene.

Poster for the November 2020 TMS Meeting (6.7 MB)