Late Pleistocene to Holocene sea-level changes in New Jersey: causes and paleoenvironmental implications
Creators: name: Johnson, Christopher Stephen
Publisher: name: No Publisher Supplied
Year: 2019

description: With locations such as Norfolk, VA, Atlantic City, NJ, and Sandy Hook, NJ experiencing ≥4 mm/yr of relative sea-level (RSL) rise over the 20th century, sea-level rise is an issue facing many coastal communities. As such, it is important to understand the factors controlling the rate of RSL rise and to be able to quantify their contributions. RSL rise is a combination of global mean sea-level change, vertical land motion (VLM), changes in Earth gravity, Earth rotation, and viscoelastic solid-Earth Deformation (GRD), and sterodynamic effects. VLM can include thermal subsidence of the lithosphere, Glacial Isostatic Adjustment (GIA), sea-level change due to Mantle Dynamic Topography (MDT), and sediment compaction (both autocompaction and groundwater induced). Sterodynamic changes involve thermosteric sea-level change, gravitational effects of changing ice-volume, dynamic topography, and changes in tidal regime. In this thesis, I attempt to quantify the local sources of VLM at Sandy Hook, NJ, and improve our understanding of the regional sources of VLM along the New Jersey margin. At Sandy Hook, the tide gauge measurements detected a twentieth century mean rate of RSL rise of 4.0±0.4 mm/yr, whereas 26 km north at The Battery, NY, the rate of RSL rise was 3.0±0.3 mm/yr for the twentieth century. The proximity of these two stations rules out most sterodynamic changes and many of the larger scale processes driving VLM as the primary driver of the diference. The major cause of the 0.9±0.5 mm/yr difference between these two points is the underlying geology. The Battery lies directly on crystalline bedrock, while Sandy Hook rests atop &gt;226 m of compressible Cretaceous to Holocene coastal plain sediments overlying bedrock. In 2014, three coreholes, drilled at Sandy Hook, revealed a thick (84+ m) Quaternary section underlying the tide gauge. As such, we hypothesized that natural compaction of this relatively young (&lt;13,350 cal yrs bp) package of sediment was the source of the “excess” subsidence detected at this location. We tested this hypothesis in Chapter 2 (Johnson et al., 2018) by creating a numerical model that simulated autocompaction through time and resulted in a 20th century average compaction rate of 0.16 mm/yr (90% Confidence Interval; C.I. 0.06-0.32 mm/yr). We then hypothesized that the remaining 0.7 mm/yr (90% C.I. 0.3-1.2 mm/yr) was due to groundwater extraction. Chapter 3 tested this hypothesis by building a groundwater model that evaluated the subsidence caused by drawdown of the groundwater under Sandy Hook and the resulting compaction. We found that groundwater extraction was responsible for 20th century average of 0.3±0.2 mm/yr of subsidence at Sandy Hook. For the duration of the tide gauge record during the 20th century the average was 0.4 mm/yr, and the current rate of groundwater related subsidence is ~0.7 mm/yr. We then compared our results to the vertical land motion measured by continually operating reference station global positioning systems, and there was generally a good qualitative agreement. The magnitudes of vertical land motion were consistent with the total of our estimates of the GIA, autocompaction, and groundwater related subsidence components of RSL at Sandy Hook. Chapter 4 examined Quaternary paleochannels on the inner continental shelf of New Jersey and their responses to GIA. We measured the incision depths of several channel systems looking for a pattern that would suggest a record of differential uplift of the region due to GIA, but the spatial coverage was insufficient to see such large-scale features. We also identified two sets of paleochannels on the inner shelf, the first older than 30 ka and trending north-south, while the younger (&lt;30 ka) set trended northwest-southeast. This suggests that GIA caused a shift in channel orientations ~30 ka and is consistent with previous work and estimates of the distribution of GIA-related tilting in the region at the time.