Philadelphia’s Diatom Archive Is a Way, Way, Wayback Machine


However, in recent decades, the sea has dominated the once-dynamic coastal margin, propelling farther inland as sea levels rise. Over the last century, the sea level along New Jersey has risen by 0.45 meters, more than double the global average of 0.18 meters. By 2100, the sea could rise by over a meter.

This dramatic rise in sea level has proven disastrous for the patchwork of marshes along New Jersey’s coastline, several of which have already succumbed to the sea. However, the full extent of the loss of these wetlands is difficult to parse because environmental monitoring only dates back a few decades.

Without a sense of a wetland’s natural conditions, ecological restoration is daunting. Having that information is crucial, says Enache. “Without [it], you are in the dark.” Thankfully, some of this missing data is recorded in the academy’s cache of diatoms.

Like most coastal margins, New Jersey is familiar with sea level rise. During the Pleistocene, when New Jersey was blanketed by ice and home to mastodons, sea ice slurped up stores of seawater. Around 18,000 years ago, sea levels sank more than 130 meters below their current levels—extending the New Jersey coastline 110 kilometers farther into the Atlantic Ocean.

The end of the last ice age sparked a steady climb in sea levels. Retreating ice sheets caused parts of New Jersey to sink. This subsidence, combined with glacial melt, proved a potent mix for rapid sea level rise, according to Jennifer Walker, a sea level researcher at Rutgers University.

In a study published last year, Walker turned to the past to put New Jersey’s current bout of sea level rise in context. “If we can understand how temperatures, atmosphere, and sea level changes are all interconnected in the past, that’s what we can use to project changes in the future.”

To gauge fluctuating sea levels over the past 2,000 years, her team examined the shells of single-celled protists called foraminifera that are finely calibrated to specific environmental conditions. This makes them a valuable proxy for reconstructing shifts in sea levels. By identifying the presence of certain foraminifera species throughout sediment cores collected from different spots along the Jersey shore, her team concluded that New Jersey’s coast is experiencing the fastest rise of sea level in 2,000 years.

The NJDEP hoped diatoms could serve as a similar tool for understanding how coastal marshes responded to the rising sea. Like foraminifera, each diatom species is extremely sensitive to environmental conditions. For example, species like the rolling-pin-shaped Nitzschia microcephala thrive in nitrogen-rich environments, making their shells a common sign of nutrient pollution. Other species, like Diploneis smithii, whose segmented shell resembles a slender trilobite, prefer saline waters. Their existence inland is a good indication of past sea level intrusion and helps researchers deduce which marshes have been prone to flooding in the past.

To pinpoint where these microscopic indicators once existed, the NJDEP deployed a team of researchers into several marshes along the coastline, ranging from heavily polluted wetlands in the north to near-pristine tidal marshes in the south. At each site, they cored into the marsh muck, sampling as deep as 2 meters in certain spots. Enache compares this method to slicing into a stack of pancakes—as you cut deeper, you are essentially going back in time from the steaming pancake just off the griddle to the soggy pancake deposited at the bottom of the stack. As they dug deeper, the researchers were traveling back decades. In total, they collected nine cores from five wetlands.


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