Abstract: The Atlantic coast of North America north of Cape Hatteras has been proposed as a “hotspot” of late 20th century sea‐level rise. Here we test, using salt‐marsh proxy sea‐level records, if this coast experienced enhanced sea‐level rise over earlier multidecadal‐centennial periods. While we find in agreement with previous studies that 20th century rates of sea‐level change were higher compared to rates during preceding centuries, rates of 18th century sea‐level rise were only slightly lower, suggesting that the “hotspot” is a reoccurring feature for at least three centuries. Proxy sea‐level records from North America (Iceland) are negatively (positively) correlated with centennial changes in the North Atlantic Oscillation. They are consistent with sea‐level “fingerprints” of Arctic ice melt, and we therefore hypothesize that sea‐level fluctuations are related to changes in Arctic land‐ice mass. Predictions of future sea‐level rise should take into account these long‐term fluctuating rates of natural sea‐level change.
Plain Language Summary: Measurements of sea‐level change have shown that during the 20th century sea‐level rise along the Atlantic coast of North America between Cape Hatteras and Nova Scotia has been faster than the global average. We investigated whether this anomaly also occurred earlier by reconstructing historical sea‐level changes from salt‐marsh sediments and microscopic salt‐marsh fossils (foraminifera). We found evidence in three locations (Nova Scotia, Maine, and Connecticut) for rapid sea‐level rise in the 18th century, which was almost as rapid as the 20th century sea‐level rise. Using additional sea‐level reconstructions from across the North Atlantic, we propose an explanation for the periods of enhanced sea‐level rise. We hypothesize that they occur during distinct phases of the North Atlantic Oscillation and during periods of enhanced ice melt in the Arctic. The fluctuations are a reoccurring feature and should be considered in planning for future sea‐level rise and coastal hazards.
3 Historical Sea‐Level Changes
The resulting three sea‐level reconstructions, plus previously published records from New Jersey (Kemp et al., 2013) and North Carolina (Kemp et al., 2011), are shown in Figure 1. To place the records in a larger‐scale geographical context, a recent record from Viðarhólmi, Iceland, is also shown (Gehrels et al., 2006; Saher et al., 2015). The record from Nova Scotia (Chezzetcook) spans a full millennium and is arguably the best‐dated sea‐level reconstruction over this time interval from any coastline in the world (70 dated levels, Table S1). The Maine and Connecticut sea‐level records span the last ~300 and ~450 yr, respectively, and help to constrain the spatial and temporal extent of recent sea‐level signals observed in Nova Scotia. The most recent part of each record is compared to nearby tide‐gauge observations obtained from the Permanent Service for Mean Sea Level (PSMSL) (Holgate et al., 2013) (Figures 1a and S7). In all cases, 20th century sea‐level trends from our proxy reconstructions agree with those from nearby tide‐gauge records in the common periods of overlap (Figure S8) demonstrating that the reconstructions accurately capture recent multidecadal‐to‐centennial sea‐level changes along these coastlines. We also compared the sea‐level reconstructions to sea‐level index points obtained from the base of the Holocene lithostratigraphic sections (Donnelly et al., 2004; Gehrels, 1999; Gehrels et al., 2005) to assess possible compaction in the sequences. If there were significant compaction in our records, we would expect the points from the basal sections, which are all located directly on a hard substrate, to plot higher than the reconstructions. However, they are in good agreement (Figure 1a), so we conclude that compaction has little impact on our sea‐level records. Partly due to spatially variable crustal motion rates controlled by GIA, the long‐term sea‐level trends differ between the sites (Piecuch et al., 2018). We adjusted the sea‐level records for GIA by removing the linear late Holocene trend for the common period between 4000 cal yr BP and 1900 CE (Engelhart & Horton, 2012), which in Nova Scotia is 1.7 mm/yr (Gehrels et al., 2005), in Maine is 0.7 mm/yr (Gehrels, 1999; Gehrels et al., 2002), and in Connecticut is 1.0 mm/yr (Donnelly et al., 2004). For the North Carolina sea‐level reconstruction, which is based on two nearby sites in Sand Point and Tump Point, we used GIA corrections of 1.0 and 0.9 mm/yr, respectively (Kemp et al., 2011). The New Jersey and Viðarhólmi records were corrected for a GIA contribution of 1.4 and 1 mm/yr, respectively (Gehrels et al., 2006; Kemp et al., 2013).The rates of the GIA‐corrected sea‐level (GCSL) reconstructions from all sites are shown in Figure 2 and are marked by two distinct features. The first feature is the 19th to 20th century GCSL acceleration, which is visible in all five North American records as well as the record from Iceland, although their exact timing and amplitude may differ between sites. This feature is also present in other salt‐marsh‐based sea‐level reconstructions from the Atlantic coast of North America (Kopp et al., 2016). The second feature is a previously unreported, multidecadal‐centennial GCSL fluctuation along the North American Atlantic coast, with maximum rates of rise occurring in the middle‐to‐late 18th century, and lower or negative rates thereafter. The timing appears to differ slightly from record to record, most likely due to dating uncertainties. Based on the 1,000 Monte Carlo ensemble members at each site, we determine for five sites best estimates for the timing of maximum rates of change, over the period 1550 to 1850, as follows: 1735 (North Carolina), 1745 (New Jersey), 1752 (Maine), 1762 (Nova Scotia), and 1783 (Connecticut). Moreover, between 85% (Nova Scotia) and 98% (New Jersey) of ensemble members show greater‐than‐zero rates averaged over the 18th century. These numbers suggest significant, larger‐than‐usual (with respect to longer‐term GIA) rates of change peaking in the middle‐to‐late 18th century. The GCSL fluctuation is more pronounced in Maine and Connecticut, compared to Nova Scotia and North Carolina. In Connecticut, GCSL rates were close to zero before ~1700 and increased to values of ~2.4 ± 2.4 mm/yr (± indicates twice the standard error) towards the end of the 18th century. High preindustrial GCSL rates are similarly visible in the Maine record (~3.2 ± 3.2 mm/yr), although the record only starts in the mid‐18th century. The New Jersey record includes the late 18th century period of enhanced sea‐level rise but overall shows greater variability than all other records. In Nova Scotia and North Carolina, rates during this period were also enhanced compared to long‐term background rates of change, but they did not exceed values of ~0.5 ± 1.0 mm/yr. It is important to note that spatial variations in the amplitudes of multidecadal sea‐level variations along this coastline are also observed in tide‐gauge records over the 20th century (Sallenger et al., 2012). Interestingly, relatively high preindustrial GCSL rates are also seen in the Viðarhólmi data from Iceland. The records from North America and Iceland are out of phase: rates of change in Iceland are anomalously low around 1700 CE and high around 1800 CE, suggesting that peak rates of preindustrial GCSL rise occurred in North America ~60–80 yr earlier than in Iceland. Possible reasons for this are considered below.
[Figure 2 Sea‐level changes from proxy records along the North American Atlantic (blue) and Icelandic (red) coast. Shown are the nonlinear trends calculated by a Gaussian Process Regression including their 1 and 2 sigma uncertainties (dark and light blue/red bands, respectively) for the six salt‐marsh reconstructions corrected for site‐specific GIA effects. The black dotted line marks a rate of 0 mm/yr in each panel.]
While the high 18th century rates of sea‐level rise are a consistent feature in our records across sites, patterns become more complex in earlier periods. For example, the New Jersey and North Carolina records are very similar, but in North Carolina sea‐level variability was more muted prior to the 18th century (a finding that is robust against different choices of the Gaussian process priors). These differences between records might be explained by different driving mechanisms or could reflect issues with the salt‐marsh reconstructions (e.g., dating resolution or quality of transfer functions). These remain open questions.
The multidecadal to centennial sea‐level fluctuations found in our records are not seen in previously published reconstructions. There are several possible explanations for this. First, some of the recently published sea‐level records from the Atlantic coast of North America (Kemp et al., 2011, 2014) are from south of Cape Hatteras and outside of the main hotspot region of the MAB identified by Sallenger et al. (2012) from tide‐gauge records over the period 1950–2009. Second, the proxy records that are from that region, i.e. New York City (Kemp et al., 2017) and eastern Connecticut (Kemp et al., 2015), lack high‐resolution data in the 18th and 19th centuries; we suggest that more detailed investigations here could reveal the same sea‐level fluctuations.
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