Investigating Holocene Hydroclimate using Sediment Cores from Florida Panhandle Sinkhole Lakes

The southeastern U.S. (SEUS) has experienced intense droughts in the last century that strain local freshwater supplies and agriculture, increase fire risk, and threaten public health. Understanding regional climate conditions that determine drought occurrence and severity is necessary to mitigate their impacts. Previous work suggests increased insolation in the mid-Holocene resulted in decreased mid-latitude precipitation at continental-scales, yet sparse hydroclimate reconstructions limits understanding of the hydroclimate response at regional scales, particularly in the SEUS. Sinkhole lakes archive a geologic record that can be used to constrain centennial and millennial-scale paleohydrologic trends in the SEUS. Geomorphometry from GPR (ground-penetrating radar) in combination with sedimentologic data from cores is useful to estimate past lake volumes and reconstruct the baseline range of natural hydroclimate variability. We estimate millennial-scale shifts in the hydrologic budget using GPR-derived transects and a ¹⁴C-based chronology of two sediment cores from Griffin Mill Pond, a 0.2 km-wide sinkhole lake 50 km inland from the Gulf of Mexico. GPR transects exhibit basin-wide high and lowstands, including truncated sediment layers and multiple onlap/offlap sequences. We rely on sediment cores to characterize the sediment layers and constrain timing of these truncations. We extracted Core One (3.5 m) from the depocenter of the lake (11.2 m water depth) and Core Two (4.4 m) from the north slope of the lake (9.5 m water depth). The base of Core One (1.7-3.5 m) is composed of siliciclastic sand, which we attribute to terrestrial sediment delivery as the lake initiated following sinkhole collapse (>17.3 ka). The sand is truncated by an erosional contact (~9.2 ka) separating it from 1.8 m of overlying organic-rich silt. We interpret the contact as a hiatus in deposition driven by decreased lake level. The silt began accumulating ~6.7 ka in Core One and continues through the present. Lake level rise is concomitant with decreased hydraulic gradients from rising sea level and evidence for an increased moisture balance from pollen reconstructions. Using this record, we identified new evidence of early-Holocene drought conditions coinciding with orbital forced insolation maxima.