Carbon cycle feedbacks during the Oligocene-Miocene transient glaciation

• Published in: Geology (Boulder) 2013-09, Vol.41 (9), p.963-966
• Main Authors: Mawbey, Elaine M, Lear, Caroline H
• Format: Article
• Language: English
• Subjects: actinides; Al/Ca; alkali metals; alkaline earth metals; aluminum; Atlantic Ocean; bottom water; C-13/C-12; calcium; carbon; Carbon cycle; Ceara Rise; Cenozoic; chemical ratios; chemostratigraphy; climate effects; cores; deglaciation; Equatorial Atlantic; Fe/Ca; Foraminifera; geochemical cycle; Geology; glaciation; Glaciers; insolation; Invertebrata; iron; isotope ratios; Isotopes; Leg 154; Li/Ca; lithium; magnesium; manganese; Marine; marine sediments; metals; Mg/Ca; microfossils; Miocene; Mn/Ca; Neogene; North Atlantic; O-18/O-16; Ocean Drilling Program; ODP Site 926; ODP Site 929; Oligocene; oxygen; paleo-oceanography; paleoclimatology; Paleogene; paleotemperature; Protista; sediments; stable isotopes; Stratigraphy; Tertiary; trace metals; U/Ca; uranium; Water temperature
• ISSN: 0091-7613; 1943-2682
• DOI: 10.1130/G34422.1

Ice sheet models suggest that once formed, the large, high-altitude East Antarctic Ice Sheet was relatively self-stabilizing, due to its cold upper surface. The ice sheet hysteresis problem results from an inability to reconcile this expectation with geological evidence for episodes of ice sheet retreat. A classic example of this problem is manifested in benthic foraminiferal oxygen isotope records across the Oligocene-Miocene boundary (ca. 23 Ma), which display a transient ∼1 per mil excursion to higher values. The inferred increase and subsequent decrease in ice volume has been linked to advance and retreat of the Antarctic ice sheet across the continental shelf. However, oxygen isotope records alone do not provide unambiguous records of temperature and ice volume, hindering assessment of the driving mechanism for these variations. Here we present new benthic foraminiferal Mg/Ca, Li/Ca, and U/Ca records across the Oligocene-Miocene boundary from Ocean Drilling Program Sites 926 and 929. Our records demonstrate that Atlantic bottom-water temperatures varied cyclically, with the main cooling and warming steps followed by ice growth and decay respectively. We suggest that enhanced organic carbon burial acted as a positive feedback as climate cooled. Several lines of evidence suggest that the deglaciation was associated with an input of carbon to the ocean-atmosphere system, culminating in a previously unidentified seafloor dissolution event. We suggest that one of the initial sources of carbon was organic matter oxidation in ocean sediments. This study demonstrates that carbon cycle feedbacks should be considered when evaluating the stability of ancient ice sheets.