REGIONAL AND LOCAL SOURCES OF RELATIVE SEA-LEVEL CHANGE AT SANDY HOOK, NJ

Geologic studies of Sandy Hook, NJ provide an opportunity to evaluate regional and local controls on relative sea level (RSL) change. 20^th century tide gauge records from Sandy Hook indicate 4.0±0.5 mm/yr of RSL rise, whereas records from The Battery, NY, 26 km NW, indicate a rise of 3.0±0.3 mm/yr. Subsidence due to Glacial Isostatic Adjustment (GIA) is 1.3±0.4 mm/yr for the 20^th century at both locations. Sandy Hook rests atop compressible coastal plain sediments, whereas The Battery overlies incompressible bedrock. We seek to explain the different subsidence histories by evaluating contributions from compaction of peats, compaction of siliciclastic muds, and anthropogenic groundwater withdrawal.

Three cores were drilled on Sandy Hook to investigate the localized sea level signal; recovering unusually thick (85+ m) sections of deglacial to Holocene (0-13,350 cal yrs) sediments. ~25 m of the section was deposited rapidly in ≤200 yrs and may be infill after a jökulhlaup (~13,350 cal yrs) that unconformably overlies Cretaceous strata. Sedimentological studies yielded high-resolution grain size, total organic carbon (TOC), and porosity data from each core. Relatively low TOC values (~4% in mud; ~1% in sand) indicate that compaction of organic material is unlikely to contribute to the local rate of subsidence. Numerical models quantify that natural compaction of deglacial muds likely reduced the column thickness by 10-20% over the past 13,350 cal yrs. Past compaction rates were tens of millimeters per year, but the 20-21^st century rate is quantified as ~0.1-0.2 mm/yr. The remaining 0.8-0.9 mm/yr difference in subsidence between Sandy Hook and The Battery is likely due to anthropogenic groundwater withdrawal. Quantifying groundwater’s contribution is complicated by poor documentation of past withdrawal rates and overlapping of local (Fort Hancock) and regional (Monmouth County) groundwater withdrawals. Initial cone of depression estimates suggest Fort Hancock’s pumping station influenced the tide gauge (~2 km NE), with additional influence from Monmouth County wells. Future groundwater modeling will attempt to quantify the contribution of groundwater withdrawal to local subsidence, though the order of contribution appears to be GIA, local/regional groundwater, and compaction of thick Quaternary muds.