Extended late Holocene relative sea-level histories for North Carolina, USA

Abstract

22We produced ~3000-year long relative sea-level (RSL) histories for two sites in North Carolina (USA) 23 using foraminifera preserved in new and existing cores of dated salt-marsh sediment. At Cedar Island, 24 RSL rose by ~2.4 m during the past ~3000 years compared to ~3.3 m at Roanoke Island. This spatial 25 difference arises primarily from differential GIA that caused late Holocene RSL rise to be 0.1-0.2 mm/yr 26 faster at Roanoke Island than at Cedar Island. However, the non-linear difference in RSL between the two 27 study regions (particularly from ~0 CE to ~1250 CE) indicates that additional local-to regional-scale 28 processes were driving centennial-scale RSL change in North Carolina. Therefore, the Cedar Island and 29Roanoke Island records should be considered as independent of one another. Between-site differences on 30 sub-millennial timescales cannot be adequately explained by non-stationary tides, sediment compaction or 31 local sediment dynamics. We propose that a period of accelerating RSL rise from ~600 CE to 1100 CE 32 that is present at Roanoke Island (and other sites north of Cape Hatteras at least as far as Connecticut), but 33 absent at Cedar Island (and other sites south of Cape Hatteras at least as far as northeastern Florida) is a 34 local-to regional-scale effect of dynamic ocean and/or atmospheric circulation. 35 36 enables contributions to RSL trends from specific processes to be estimated (or indeed discounted) by 48 comparing and contrasting RSL histories across a suite of sites (e.g., Long et al., 49 2014). The most precise RSL reconstructions are generated in depositional environments with small tidal 50 ranges . 51 52 Estuaries enclosed by the Outer Banks of North Carolina (USA; Figure 1) are ideal places to produce late 53Holocene RSL reconstructions because the region is characterized by great diurnal tidal ranges (mean 54 lower low water, MLLW, to mean higher high water, MHHW) that are often less than 0.2 m and their 55 expansive salt marshes are underlain by thick and continuous late Holocene sequences of salt-marsh peat. 56

Sea Level Rise Explains Changing Carbon Accumulation Rates in a Salt Marsh Over the Past Two Millennia

“…While the RSLR data from Kemp et al () extend to the year 2005 CE, RSLR monitored near the Traps Bay study site show consistently accelerating rates of RSLR. From 2008–2016 CE, areas within 5 km of Traps Bay exhibited RSLR rates as high as 10 and 14 mm/year, which greatly exceed the long‐term local average of 3.0 mm/year and is consistent with a Juncus to Spartina transition (Currin et al, ).…”

“…These age‐depths cannot be translated into a precise sea level reconstruction since foraminiferal assemblages or another biostratigraphic proxy needed to reconstruct the paleomarsh elevation within the tidal range were not identified. Instead, we inferred past RSLR for this area from a reconstruction completed in a nearby study site (Kemp et al, ). The age‐depth horizons independently determined by Kemp et al () using 14 C and 210 Pb agreed remarkably well with ours (Table ).…”

“…Instead, we inferred past RSLR for this area from a reconstruction completed in a nearby study site (Kemp et al, ). The age‐depth horizons independently determined by Kemp et al () using 14 C and 210 Pb agreed remarkably well with ours (Table ). Therefore, we justified using the same RSLR estimates from the Kemp et al study site (Cedar Island) at Traps Bay and using the depth‐ages determined by Kemp et al () to date the bases of Cores 2L and 3L.…”

“…Age was inferred by matching depth of marsh contact to geochronology produced in Kemp et al (). See section .…”

“…It is well known that net sediment accumulation rates decrease with increasing time span across subtidal and terrestrial depositional environments mainly due to discontinuous sedimentation (Sadler, 1981). Sediment accumulation of salt marsh strata, however, has been shown by many researchers to be continuous over decadal to millennial timescales (e.g., Gehrels, 1999;Kemp et al, 2017;van de Plassche et al, 1998) mainly due to salt marshes having a high resistance to erosion (Neumeier & Ciavola, 2004) and a strong positive relationship between accretion and inundation time (Morris et al, 2002;Pethick, 1981).…”