SHORT-LIVED RADIOISOTOPES AS GEOCHRONOMETERS IN EVENT STRATIGRAPHY

Event stratigraphy, detecting singular events in the stratigraphic record, has gained recent popularity, primarily in an effort to assess human impacts to natural systems. This requires linking a known event (e.g. tropical cyclone, anthropogenic activity) to a specific sedimentary response (i.e. “signature”), which in turn requires accurate, high-resolution, chronologically recent dating techniques. Once an event “signature” is identified, its occurrence in the sedimentary record can be used to determine the natural system variability, which can then be used to gauge anthropogenic impacts. Short-lived radioisotopes (SLR) are often the geochronological tool(s) of choice in that they apply to the timeframe of greatest anthropogenic impacts, and can produce high-resolution results. Often overlooked, but equally as important, is the stratigraphic integrity of the sedimentary record sampled, including sediment homogeneity (texture/composition), extent of mixing, accumulation rates, and coring/extrusion techniques. In short, knowing where and how to sample.

These points are illustrated in examples from the US Virgin Islands (USVI) and NE Gulf of Mexico (GOM) following the 2010 BP oil spill. Typical USVI coastal sediments are mixed, whereas sediments from coastal salt ponds exhibit well-preserved, mm- to cm-scale laminations representing episodic input from heavy rainfall/runoff, alternating with overwash (tropical cyclone, tsunami) events. Stratagraphically, periods dominated by rainfall/runoff alternate with periods dominated by overwash (from tropical cyclones) on a decadal scale, but the record over the past ~2ky, shows the cyclicity has fluctuated. Cores collected from the NE GOM following the BP oil spill provide a unique opportunity to study the sedimentary response to a major event in real time. Sediments contain an internally stratified, organic-mud surface layer, all deposited within 6 months following the spill. Due to the rapid accumulation, ²³⁴Th, typically used for bioturbation studies (half-life ~24 days), exhibits a down-core decay profile that can be used as a geochronometer. Mass accumulation rates show an increase of up to 100x over pre-event rates. ²³⁴Th profiles ~1 year following the event are closer to “natural”, suggesting a return to more normal sedimentary conditions.