Collaborative Research: Partitioning early Holocene Laurentide v. Antarctic ice melt from high-resolution reconstruction of sea-level rise and glacial isostatic adjustment modeling

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Sea-level rise is a defining issue for the future of densely populated low-lying coastal regions. Within this context, the fate of the remaining large ice sheets is arguably the primary concern. Therefore, numerous studies over the past few decades have focused on the retreat of ice sheets during the end of the last ice age, 20,000 to 7,000 years ago. The North American and Antarctic continents collectively account for up to 80% of total ice melt, but the amount and timing of their contributions is not well known. Resolving this is important because the location of meltwater input is likely to have major implications for ocean-atmosphere circulation across the globe. The period between 12,000 and 7,000 years ago that witnessed about half (the last ~60 m) of the total sea-level rise is particularly poorly studied, despite the fact that the Earth's climate was roughly comparable to that of the present-day. Thus, this time interval could serve as a useful analog for the future. The present project will reconstruct the rate of sea-level rise in the Mississippi Delta during this period with unprecedented resolution. Combining this new record with state-of-the art computer models of ice sheets and the solid earth, it will be possible to calculate the relative contribution of North American v. Antarctic ice sheets to global sea-level rise during this time interval. The project will support a postdoctoral scholar as well as undergraduate students, and also contain numerous outreach activities. This includes instruction and field demonstrations at a New Orleans urban farm that educates local youth leaders on sustainable food production. Educational materials will be provided for high school teachers throughout Florida and for an exhibit on sea-level rise in the Tampa Bay-St. Petersburg area. Finally, all new sea-level data will be publicly released through the NOAA-WDC Paleoclimatology database.Uncertainty about the meltwater contribution from different ice sheets during the last deglaciation is particularly well reflected by the sea-level history during the early Holocene, where recent end-member estimates of the Laurentide Ice Sheet (LIS) contribution to global sea-level rise between 11 and 7 ka vary from ~20 to 30 m sea-level equivalent. This difference corresponds to about 1.5 times the volume of the present-day Greenland Ice Sheet. Possibly even larger uncertainties exist about the retreat history of the Antarctic Ice Sheet (AIS). Ice-sheet volumes during this time window are equally poorly constrained. The Mississippi Delta is uniquely suited to produce a highly detailed early Holocene relative sea-level (RSL) record, extending to ~9.5 ka and possibly older. Wherever conventional approaches (AMS 14C dating of plant macrofossils extracted from basal peat) are not feasible, ramped pyrolysis 14C dating of brackish wetland paleosols, a novel geochronological technique, will be used to obtain sea-level index points of sufficiently high resolution. The new early Holocene RSL record will be combined with a synthesis of lower resolution records from across the globe, with the primary goal to determine the rate of RSL rise and its spatial variability from 7 to 10 ka. Glacial isostatic adjustment modeling that considers a large suite of ice-sheet histories will be used to correct RSL records for viscoelastic and rotational effects. The residual signal should provide new information on the contribution from the LIS and AIS to early Holocene sea-level rise, due to their distinctly different sea-level fingerprints. Specific questions that the work will answer include: (1) Was the early Holocene sea-level rise predominantly smooth or was it punctuated by distinct sea-level jumps akin to (and possibly larger than) those around the 8.5 to 8.2 ka time window? and (2) What was the relative contribution of the LIS v. AIS to early Holocene sea-level rise, in particular during the time interval from ~9.5 to 8 ka?