Coupled change in global climate and the carbon cycle across the Eocene-Oligocene transition: New insight from the Pacific Ocean, IODP Exp 320

Carbon dioxide, CO2, is a powerful greenhouse gas and its concentration in Earth's atmosphere has increased by around 35% since the start of the Industrial Revolution (in a ca. 250 yr time period) to a level that is higher than at any time in the past 800 thousand years as measured in air bubbles from ice cores. If man-made (anthropogenic) CO2 emissions to the atmosphere follow projected rates, then by 2100, concentrations will reach values not seen on Earth since the Palaeogene epoch (ca. 65-23 million years ago, Ma). These startling observations mean that we must improve our understanding of Palaeogene climate. Arguably, the pivotal Palaeogene climate event occurred around 34 Ma when, the first large Antarctic ice sheets were rapidly established across the Eocene/Oligocene transition, EOT. Results of ice-sheet-climate model experiments indicate that this event might represent a 'tipping point' response to slow decline in atmospheric CO2 levels (many orders of magnitude slower than the rate of anthropogenic increase). The results of our own previous research show that the growth of ice sheets on Antarctica across the EOT occurred in lock-step with a deepening (by more than a kilometer) in the calcite compensation depth (CCD) in the tropical Pacific Ocean. The CCD is the depth at which calcium carbonate sediments are dissolved in the ocean and can be thought of as 'an ocean acidity indicator'. Ocean de-acidification across the EOT demonstrates that the switch from a largely non-glaciated 'greenhouse' world to one with large ice sheets on Antarctica was closely associated with a big disruption in the global carbon cycle. What linked ice sheet growth on Antarctica to the acidity of the tropical deep Pacific Ocean? Previously we have suggested that growth of ice sheets on the Antarctic continent causes sea level to fall, killing off the large expanse of calcium carbonate (CaCO3)-secreting reefs that previously flourished on the continental shelves. This would have caused an imbalance between the inputs (from rivers) and outputs (sediment burial) of dissolved CaCO3 to the global ocean requiring a deepening in the CCD until balance was restored through increased CaCO3 sedimentation in the deep ocean. In numerical 'box model' tests, this 'shelf-to-basin CaCO3 switching' mechanism performed better than competing mechanisms proposed to explain EOT events. Yet shelf-basin-switching has been called into question on a number of grounds. For example, it has been suggested that the carbon cycle changes across the EOT were driven by changes in surface ocean productivity. In 2009, Integrated Ocean Drilling Program Expedition 320 drilled a series of holes across the Pacific Ocean on ocean crust of different age. The EOT was captured in four new sites at successively shallower depths. Our plan is to compare the chemistry of this EOT 'age-depth transect' of new sites with our existing records from a latitudinal transect of sites drilled previously in the region (ODP Leg 199). Hypotheses for the EOT that invoke CCD deepening in response to changes in global deep ocean carbonate chemistry (eg. shelf-basin-switching) should produce a pattern of sedimentation that is a simple function of depth whereas we predict the imprint of latitude from changes in surface productivity. One key to our work is the potential for unprecedented age-control in our target sediments. Other key aspects are the availability of two new sites that are richer in CaCO3 and calcareous microfossils across the EOT (specifically the latest Eocene) than the best previously available Pacific site (ODP 1218) and new geochemical techniques for determining changes in carbonate chemistry of the ancient oceans.