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

In recent decades, however, the sea has dominated the once dynamic coastline, encroaching ever further inland as sea levels rise. Over the past century, sea level has risen 0.45 meters along New Jersey, more than double the global average of 0.18 meters. By 2100, sea levels could rise by more than a meter.

This dramatic sea level rise has proved disastrous for the patchwork of swamps along the New Jersey coast, some of which have already fallen to the sea. However, the full extent of the loss of these wetlands is difficult to analyze because environmental monitoring dates back only a few decades.

Without a sense of a wetland’s natural conditions, ecological restoration is daunting. Having this information is critical, says Enache. “Without [it]you’re in the dark.” Fortunately, some of this missing data is recorded in the Academy’s diatom cache.

Like most coasts Margen, New Jersey is familiar with sea level rise. During the Pleistocene, when New Jersey was covered in ice and home to mastodons, sea ice slurped up supplies of seawater. Around 18,000 years ago, sea levels dropped more than 130 meters below their current level, extending the New Jersey coastline 110 kilometers further into the Atlantic Ocean.

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The end of the last ice age triggered a steady rise in sea levels. Retreating ice sheets caused parts of New Jersey to sink. That subsidence, combined with glacial melt, proved to be an effective 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 current sea-level rise in New Jersey into context. “If we can understand how temperatures, atmosphere and sea level changes are linked in the past, we can use this to predict changes in the future.”

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To measure sea-level fluctuations over the past 2,000 years, her team studied the shells of single-celled protists called foraminifera, which are finely tuned to specific environmental conditions. This makes them a valuable proxy for reconstructing sea-level shifts. By identifying specific foraminiferal species in sediment cores collected from various locations along the Jersey shore, her team concluded that the New Jersey coast is experiencing the fastest sea-level rise in 2,000 years.

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

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To determine where these microscopic indicators once existed, the NJDEP dispatched a team of researchers to several swamps along the coast, ranging from heavily polluted wetlands in the north to nearly pristine tidal marshes in the south. At each point, they drilled cores into the swamp mud and took samples up to 2 meters deep at certain points. Enache likens this method to slicing into a stack of pancakes—if you cut deeper, you’re essentially going back in time, from the steaming pancake straight off the griddle to the soggy pancake set down at the end of the stack. As they dug deeper, the researchers traveled back decades. In total, they collected nine drill cores from five wetlands.

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