Catastrophic hurricane history revealed by organic geochemical proxies in coastal lake sediments: a case study of Lake Shelby, Alabama (USA)

摘要

We developed a new method for reconstructing millennia-long hurricane records from coastal environments that uses Organic Geochemical Proxies (OGPs) of organic carbon and nitrogen concentrations and their delta C-13 and delta N-15 compositions. The new method is independent of presence/absence of sand layers and improves significantly the severe-storm history resolution. The subject of this investigation is a 1.5 m long sediment core raised at 2.8 m water depth from the center of Lake Shelby, Alabama, a freshwater lake located approximately 250 m from the Gulf of Mexico, from which an overwash sand-layer based record was previously derived. The core contains two distinct sediment units; an upper 62 cm thick, fine-grained, organic-rich lacustrine sapropel (gyttja) that shows no visible structures except one sand lamina at 23.7 cm depth, and an underlying 90 cm thick, organic-poor lagoon/estuary clay unit. The sapropel unit was deposited over a 682 +/- 30 cal year time interval (1320-2002 A.D.) with a mean sedimentation rate of 0.79 +/- 0.04 mm/year. Lake Shelby's water column exhibits two contrasting states based on water chemistry surveys (i) an "isolated", stratified, mode under calm weather conditions with a relatively low trophic state, and (ii) a "flooded" mode occurring during storm surges when nutrient-rich seawater floods the lake. Statistically significant delta C-13 and delta N-15 positive excursions in organic matter, up to maximum values of -25 (parts per thousand PDB) and 4 (parts per thousand Air N-2), respectively, are interpreted as geochemical responses to the marine intrusions that fertilize the lake, increase light availability, and cause eutrophication spikes. Detailed OGPs analyses crossing a sand layer that offers visual evidence of a catastrophic hurricane overwash event at 1717 A.D. exhibit large delta C-13 and delta N-15 positive shifts bounded by rapid returns to base values, thus confirming the validity of the hurricane identification by the OGPs model. Our data indicate that 11 catastrophic hurricanes hit the Alabama coast over the past 682 years with a rough recurrence interval of one in 62 years.