Silicate weathering and carbon cycle controls on the Oligocene-Miocene transition glaciation

• Published in: Paleoceanography
• Main Authors: Stewart, Joseph A., James, Rachael H., Anand, Pallavi, Wilson, Paul A.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2017
• Subjects: Antarctic; The Antarctic; Antarc*; Ice Sheet; Planktonic foraminifera
• Online Access: https://oro.open.ac.uk/51664/; https://oro.open.ac.uk/51664/2/Stewart_et_al-2017-Paleoceanography.pdf; https://doi.org/10.1002/2017PA003115

Changes in both silicate weathering rates and organic carbon burial have been proposed as drivers of the transient “Mi-1” glaciation event at the Oligocene-Miocene transition (OMT; ~23 Ma). However detailed geochemical proxy data are required to test these hypotheses. Here we present records of Li/Ca, Mg/Ca, Cd/Ca, U/Ca, δ 18 O, δ 13 C, and shell weight in planktonic foraminifera from marine sediments spanning the OMT in the equatorial Atlantic Ocean. Li/Ca values increase by 1 μmol/mol across this interval. We interpret this to indicate a ~20% increase in silicate weathering rates, which would have lowered atmospheric CO 2 , potentially forcing the Antarctic glaciation circa 23 Ma. δ 13 C of thermocline dwelling planktonic foraminifera track the global increase in seawater δ 13 C across the OMT and during the Mi-1 event, hence supporting a hypothesized global increase in organic carbon burial rates. High δ 13 C previously measured in epipelagic planktonic foraminifera and high Cd/Ca ratios during Mi-1 are interpreted to represent locally enhanced primary productivity, stimulated by increased nutrients supply to surface waters. The fingerprint of high export production and associated organic carbon burial at this site is found in reduced bottom water oxygenation (inferred from high foraminiferal U/Ca), and enhanced respiratory dissolution of carbonates, characterised by reduced foraminiferal shell weight. Replication of our results elsewhere would strengthen the case that weathering-induced CO 2 sequestration preconditioned climate for Antarctic ice sheet growth across the OMT and increased burial of organic carbon acted as a feedback that intensified cooling at this time.