Radiocarbon Age Offsets Among co-Existing Foraminifers: Effects of Bioturbation, Dissolution, and Sediment Redistribution

Abstract:

Radiocarbon dating of microfossils in deep sea cores has many caveats. Of these, three are the most problematic: bioturbation, dissolution, and lateral sediment redistribution. We attempted to isolate these caveats by selecting cores with different sedimentation rates and water depths. Our ultimate goal is to identify a foraminifer species that is affected the least by these caveats and thus is the most reliable for age dating. We present species-specific radiocarbon ages, whole assemblage counts, and calculations of sediment mixing rates in two cores extending to the Last Glacial Maximum (LGM): Y69-71 in the Panama Basin and MD98-2177 in the Indo-Pacific Ocean. Thorium normalization yields high lateral sediment redistribution in Y69-71 which is located below the lysocline. MD98-2177 has high sediment accumulation rates (~50cm/kyr) and is located above the lysocline. We found large disagreement among the ages of co-occurring microfossils in several horizons (up to 2000 years) in MD98-2177. Based on our whole assemblage counts bracketing these horizons, we conclude that bioturbation is the most likely agent of age discrepancy among co-occurring sedimentary components in MD98-2177 despite its high sedimentation rate. Surprisingly, the age offset among co-occurring components is reduced in Y69-71, especially during the LGM when the sediment focusing factor was about 8. This may suggest that sediment focusing was not disruptive enough to hinder the accurate dating of sediment layers in this core, especially during the LGM when foraminifer preservation was good. Lastly, we conclude that shells of G. menardii may be the most reliable for dating due to their flat morphology and high dissolution resistance.