HYBLA CORES 7 & 8: AN 80,000-YEAR LATE PLEISTOCENE CLIMATE RECORD FROM THE MID-ATLANTIC COASTAL PLAIN OF NORTH AMERICA

In 2008, we collected 8 cores in Hybla Valley (Fairfax Co., VA) to investigate Late Pleistocene sediments flanking the Potomac River Valley. Taken from beneath the ~15 m asl elevation surface, Hybla 7 recovered 35 m of sand, silt, and mud before it hit core refusal in a pebble conglomerate. When composited with a shallower adjacent core (20 m, Hybla 8) we obtained nearly complete recovery to 35 m depth. Radiometric ages derived from 1 accelerator mass spectroscopy (AMS) ¹⁴C sample and 4 optically stimulated luminescence (OSL) samples resolved a 2^nd order polynomial age-depth model (R²=0.97) that indicated ages of ~145 ka to ~65 ka for the core interval between 35 m and 10 m depth.

The interval from 34-10 m is dominated by mud and silt, and was analyzed at 20 cm resolution for palynomorphs. Sedimentary features suggest that accumulation was almost continuously subaqueous with intervals of interlayered sand and peat, and a deep soil developed from 12-10 meters. Pollen assemblages exhibit marked shifts in tree pollen composition and abundances, which we attribute to dynamic responses to changes in mean annual temperature. These shifts are of sufficient resolution that, by using our age-depth chronology, the pattern of pollen change (i.e., proxy temperature) displays a strong similarity to the patterns and timing of the oxygen isotope curve (i.e., proxy temperature) in the Greenland ice cores over 3700 km away. Our record matches the timing and nature of the ice core thermal shifts, even for short-lived events (Greenland Interstadials (GIS) 21-24). Our interpretation is that the analyzed interval spans the end of oxygen isotope stage (OIS) 6 to the initiation of OIS 4. To compare dominant forest end-members, OIS 4 was dominated by spruce, OIS 5c by bald cypress, and OIS 5e by oak and tupelo at the core site. One subinterval (OIS 5e3) also contained dinoflagellates, indicating direct connection to the tidal Potomac and requiring near-marine salinity.

We conclude that the core interval analyzed here represents ~80,000 years of nearly continuous sedimentation that varied in rate as a function of climate, and that contains only temporally minor (probably centennial-scale) hiatuses. This Hybla 7&8 core set is an important new paleoclimate datum in a geographic region that is notably under-represented by long climate records.