Guest geology by David Middleton
Ghost Forests Are Visceral Examples of the Advance of Climate Change
BY TIK ROOT OCTOBER 7, 2019
As Matt Kirwan walks through Maryland’s Blackwater National Refuge, his rubber boots begin to squish. With each step the land beneath him turns from dry ground to increasingly soggy mud. The trees around him go from tall and full of leaves or needles to short, bare and pale white.
Partway out, ankle deep in water, Kirwan stops. “At this point we’ve transitioned from being in the forest, to actually being in a full-fledged marsh,” explains the Virginia Institute of Marine Science ecologist. “This ground is now too salty and too wet to support living trees.”
Kirwan is standing in the midst of what is known as a “ghost forest.” These swaths of dead, white, trees are created when salty water moves into forested areas, first slowing, and eventually halting, the growth of new trees. That means that when old trees die, there aren’t replacements.
Kirwan likens ghost forests to other drastic markers of environmental change. Similar to how a receding glacier leaves signs of where ice used to be, ghost forests represent where dry land used to be. “You can touch it and see it,” he says. “It’s just as real as a melting glacier.”
“Ghost forests are the most striking indicator of climate change anywhere on the Atlantic coast,” says Kirwan.
One of the primary mechanisms for ghost-forest creation is sea-level rise, says Keryn Gedan, a professor at George Washington University in D.C., and Kirwan’s co-author on a recent article about the phenomena in the journal Nature Climate Change.
Climate change also exacerbates events such as hurricanes, which bring storm surges that drive salt-water landward, explains Emily Bernhardt, a professor at Duke University. Droughts, which have been similarly linked to climate change, mean less rain to wash out the salt that enters the ecosystem, “compounding the effects of sea level rise,” she says.
The word “subsidence” was notably not used at any point in this article.
Tik Root has a a BA in International Politics and Economics from Middlebury College (a Bill McKibben brainwashing victim?). He has an excuse… He’s not a scientist. What about the professors who contributed to this article? The paper by Kirwan & Gedan at least mentions subsidence. Although they never mention the fact that Blackwater National Wildlife Refuge, located on the eastern shore of Chesapeake Bay, is an area notorious for land subsidence.
More than half of the sea level rise in the vicinity of Blackwater National Wildlife Refuge is due to subsidence.
Pleistocene relative sea levels in the Chesapeake Bay region and their implications for the next century
Today, relative sea-level rise (3.4 mm/yr) is faster in the Chesapeake Bay region than any other location on the Atlantic coast of North America, and twice the global average eustatic rate (1.7 mm/yr). Dated interglacial deposits suggest that relative sea levels in the Chesapeake Bay region deviate from global trends over a range of timescales. Glacio-isostatic adjustment of the land surface from loading and unloading of continental ice is likely responsible for these deviations, but our understanding of the scale and timeframe over which isostatic response operates in this region remains incomplete because dated sea-level proxies are mostly limited to the Holocene and to deposits 80 ka or older.
STUDY SITE AND METHODS
To reconstruct the sea-level history in Chesapeake Bay, we focused on the Blackwater National Wildlife Refuge (~110 km2; red-bordered rectangle on Fig. 1), which experienced major inundation and transformation of wetlands to open water in the twentieth century (Fig. 3). Sediment from 70 boreholes was described, analyzed, and sampled. The DEM (Fig. 4) was used to characterize the geomorphology. We constrained the oldest erosional event preserved directly above the underlying Miocene strata using cosmogenic nuclide isochron burial dating (Balco and Rovey,
2008). We dated 28 samples using optically stimulated luminescence (OSL) dating. The OSL ages allow us to develop a geochronological framework for the Blackwater National Wildlife Refuge landforms and estuarine sediments to a depth of ~9 m (Fig. 5). Eight radiocarbon dates constrain the timing of Holocene inundation and the beginning of marsh accretion. Detailed methods are provided in the online GSA Supplemental Data Repository.
Our data support the hypothesis that subsidence in the Chesapeake Bay region is caused by the continued collapse of the MIS 2 forebulge (Potter and Lambeck, 2003). While subsidence rates vary within the Chesapeake Bay region (Fig. 1) (Engelhart etal., 2009), potentially due to local groundwater withdrawal for commercial use (Eggleston and Pope, 2013), the central Delmarva Peninsula has the highest rates of subsidence in the mid-Atlantic region (~1.3–1.7 mm/yr; Engelhart et al., 2009). Parsing
GIA-driven subsidence from other RSL drivers is uncertain (e.g.,
Cronin, 2012), but the agreement of twentieth-century subsidence
values calculated from tide gauge records where effects of seasonal
and decadal variability are removed (~1.6 mm/yr, Boon et al., 2010) and from dated Holocene deposits (~1.3 mm/yr; Engelhart et al., 2009) from the same location near our study area implies consistency of rates over millennial timescales. Subsidence is thus primarily driven by GIA in the Chesapeake Bay region, which makes RSL rise in the Chesapeake Bay–Washington D.C. area twice the twentieth-century global average rate of sea-level rise (1.7 mm/yr; IPCC, 2013). If timescales of MIS 6 forebulge subsidence are used for comparison, subsidence from the LGM forebulge collapse will continue for many more millennia.
[…]DeJong et al., 2015
At this point the authors went off on a tangent about accelerating sea level rise in the area, citing the IPCC and other nonsense. However, up until that point, this was some solid geology.
This series of sea level rise curves tells the story quite well.
Actual measurements of land elevation changes southwest of Blackwater NWR are consistent with the GIA model.
This section from DeJong et al., 2015 is baffling…
These are minimum estimates; several lines of evidence suggest that sea levels will rise more quickly in the Chesapeake Bay region. Recent tide gauge analyses indicate the acceleration of sea-level rise in the North Atlantic in recent decades, possibly due to dynamic ocean circulation processes (Yin et al., 2010; Boon, 2012; Ezer and Corlett, 2012; Sallenger et al., 2012).DeJong et al., 2015
There are two NOAA tide gauges stations in the vicinity:
Neither station exhibits any indication of acceleration.
The Ghost Forests of the Delmarva Peninsula “are visceral examples of climate change”… at the end of the Pleistocene Epoch. At least half of the apparent sea level rise in the area is due to subsidence of the land. While some of the subsidence is due to groundwater withdrawal and some possibly related to the underlying 35 Ma Chesapeake impact crater, the vast majority of it appears to be due to the “continued collapse of the MIS 2 forebulge”, which is due to the retreat of the Laurentide Ice Sheet at the end of the final glacial stage of the Pleistocene Epoch.
DeJong, Benjamin & Bierman, Paul & Newell, Wayne & Rittenour, Tammy & Mahan, Shannon & Balco, Greg & Rood, Dylan. (2015). Pleistocene relative sea levels in the Chesapeake Bay region and their implications for the next century. GSA Today. 25. 4-10. 10.1130/GSATG223A.1.
Eggleston, Jack, and Pope, Jason, 2013, “Land subsidence and relative sea-level rise in the southern Chesapeake Bay region: U.S. Geological Survey Circular 1392”, 30 p., https://dx.doi.org/10.3133/cir1392.
Kirwan, Matthew & Gedan, Keryn. (2019). “Sea-level driven land conversion and the formation of ghost forests”. Nature Climate Change. 9. 450-457. 10.1038/s41558-019-0488-7.