EASTERN EQUATORIAL PACIFIC CACO3 DISSOLUTION DURING THE MID-MIOCENE: A STUDY USING IODP PEAT CORES

X-ray Fluorescence (XRF) scan data from Integrated Ocean Drilling Program (IODP) Expedition 320/321 in the Eastern Equatorial Pacific show evidence of carbonate dissolution occurring at multiple times during the Cenozoic. These large-scale changes in the carbon cycle are reflected in changes of the carbonate compensation depth. We have identified a major carbonate dissolution cycle during the early to mid-Miocene (~18-16 Ma). Inadequate core recovery within the interval has hindered study of this 17 Ma dissolution event. IODP Expedition 320/321 (Pacific Equatorial Age Transect [PEAT]) recovered new, nearly continuous sediment records of the early Miocene allowing much more detailed study.

XRF scan data from Sites U1335, U1336, and U1337 over the 18-16 Ma interval and XRF scan data from Deep Sea Drilling Program (DSDP) Site 574 are used to better define the timing and understanding of how productivity and changes in ocean carbon affected carbonate preservation in the sediments. The 17 Ma event lasted ~1 million years and is strongly influenced by orbital insolation changes. The CaCO₃ analyses indicate cyclicity on an approximately 100,000 year time scale.

Core sections from PEAT Sites U1335, U1336, and U1337, and DSDP Leg 85 Site 574 were analyzed at 2.0 to 2.5 cm intervals to provide elemental identification of sediment composition in the dissolution interval. Ca and Si are used to calculate biogenic carbonate and silica, while the majority of Ba is biogenic Ba, which is an effective proxy for productivity. Comparison of the different signals allows us to determine how the interval was affected by both productivity and dissolution. %C, %CaCO₃, %C_org, and %Si analyses were completed in order to develop a calibration curve for the U1337 XRF data to approximate the amount of each of those components within the cores.

We use all three of the IODP Leg 320/321 sites in the study to provide a matrix of sedimentation rates and latitude for the early Miocene. These sites were at roughly the same depth at 17 Ma, ~4 km, and formed a transect of paleolatitude from the equator, through 1°N to 3.5°N. Since the magnitude of productivity strongly depends on latitude in the equatorial Pacific, these three sites can be used to distinguish between productivity transients and dissolution events.