A NEW RECORD OF HOLOCENE PALEO-FLOODS IN MOBILE BAY, AL FROM GRAIN SIZE, ELEMENTAL, AND FORAMINIFER ANALYSES

Flood events in coastal estuaries cause salinity changes, increase nutrient and sediment flux, and can trigger algal blooms that may lead to bottom water hypoxia. To evaluate anthropogenic influences on coastal ecosystems, we provide a long-term, natural flood record prior to settlement and industrialization. We developed multi-proxy paleo-flood assessment tools for sediment archives in coastal estuaries, and these tools are used to evaluate the magnitude and frequency of riverine floods in Mobile Bay over the last ~8,000 yrs.

Piston Core MB 18-01, collected in north-central Mobile Bay, recovered 8.39 m of Holocene bay mud. Peaks in sand concentrations from laser particle size analysis indicate paleo-flood events. Elemental composition from continuous handheld XRF measurements, including detrital Ti, K, and Zr, corroborates sand-rich layers interpreted as paleo-floods. Ca and Sr are proxies for marine influence and may also be associated with marine storm events. Likewise, nutrient proxies of Fe, Cu, and Ba are linked to the nutrient loading that may be associated with HABs. Foraminifer assemblages from the upper 200 cm of MB18-01 suggest potential hypoxic events and may elucidate nutrient and oxygenation relationships with floods.

A sediment accumulation rate of 1 cm per 10 years is assumed following published age models of nearby cores. Mobile Bay flooded ~8.2 cal kyr BP during rapid sea-level rise and the Mobile Bayhead Delta formed. Mobile Bay sustained quiescent, stable conditions and slow transgression from the beginning of our record 5.9 cal kyr BP to 5.0 cal kyr BP. From 5.0 to 2.7 cal kyr BP, repeated terrestrial floods linked to hurricane events impacted the bay, and an oyster reef developed in the central bay. From 2.7 cal kyr BP to present, fewer storms and floods appear to have impacted northern Mobile Bay. From 1.7 to 1.5 cal kyr BP, one potential nutrient loading event is recorded, and at least one major storm is recorded sometime between 1270 to 1670 CE, which may be consistent with historical records. Further analysis will evaluate oxygenation and salinity through foraminifer assemblages.