Exploring the late Holocene sedimentary record of the Mississippi Delta for climate/sea level connections
The main outcome of this dissertation is a highly detailed RSL record constructed for the time interval 600 to 1600 AD to explore the eustatic response of sea level to the atmospheric warming that occurred during the Medieval Warm Period (∼900 to ∼1200 AD) and the transition into the Little Ice Age (∼1400 to ∼1900 AD). This new record was obtained in the microtidal Mississippi Delta and contains 28 sea-level index points that track ∼60 cm of relative sea-level rise. Basal peat was used as a tracer of sea-level, and age control was obtained by AMS 14C dating. All data were plotted as error boxes using 2 sigma confidence levels. A clustering of index points between 1000 and 1200 AD suggests an increase in the rate of sea-level rise. While this suggests a fluctuating eustatic sea level, any centennial-scale fluctuation could not have had an amplitude larger than ~30 cm. Rates of sea-level change calculated from the first derivative of a third-order polynomial function fitted through the central point of each error box (r2=0.96) indicate that the maximum rate of sea-level rise occurred around 1100 AD, which slightly postdates peak Medieval warmth according to most Northern Hemisphere paleotemperature records. The dominant control of the long-term rate (0.56 mm yr-1) of RSL rise obtained from the new reconstruction is glacial isostasy in the form of forebulge collapse. Subtracting the rate of eustatic sea level rise for the 20th century (1.7 mm yr-1) (Church and White, 2006) from the rate of sea level rise captured by the Pensacola (Florida) tide gauge (2.2 mm yr-1), an area that is considered tectonically stable, yields a residual of 0.5 mm yr-1. This figure is in agreement with the long-term rate obtained in this study. This value is also very close to what some geophysical models predict as the present-day rate of glacial isostatic adjustment for the north central Gulf Coast. Taking the ratio of the instrumental rate for the last 90 years captured by the Pensacola tide gauge (2.2 mm yr-1) to the long-term geologic rate (0.56 mm yr-1) indicates a four-fold increase in the rate of relative sea level rise during the 20th century compared to that of the previous millennium. Given that at time scales of one-thousand years processes like glacial isostasy and tectonic subsidence behave essentially linearly, and that both records are free of compaction, the increase can only result from an acceleration in the rate of global sea level rise.