Corals, turfgrass and sediments offer stories of climate past and future

Science News reports from San Francisco at the fall meeting of the American Geophysical Union

Hawaiian corals could provide ancient rainfall record

REMAINS OF THE DAY The parallel, closely spaced bands on some of these ilmenite crystals are evidence of a high-pressure shock from an extraterrestrial impact. These crystals suggest such an impact occurred in the Atlantic Ocean about 2,300 years ago near what is now New York City, swamping the shore with an immense tsunami. IMAGE CREDIT: D. Breger, Drexel University

SAN FRANCISCO — Corals off the shore of a Hawaiian island may serve as monitors of groundwater flow and past climate in the region.

As the coral skeletons grew, they likely chronicled changes in the volume of groundwater seeping into the shallows offshore. Those changes, in turn, reflect variations in the island’s rainfall.

A coral colony’s hard parts are mostly made of calcium carbonate, but as the creatures grow they also incorporate into their skeletons other elements dissolved in seawater, says Nancy Grumet Prouty, a geochemist at the U.S. Geological Survey’s Pacific Science Center in Santa Cruz, Calif. The element yttrium appears in low concentrations in seawater but is common in the groundwater of Molokai, one of the Hawaiian Islands.  As rain falls and seeps into the ground, the groundwater accumulates and flows into the ocean, just as the island’s surface streams do. The groundwater, though, picks up yttrium on the way.

When Grumet Prouty and her colleagues analyzed corals collected between one and two kilometers off the southern shore of the island, they found that variations in the concentration of yttrium incorporated into the colonies in recent years tracked fluctuations in the island’s rainfall — replicating measures obtained from the average flow of the island’s streams.

In 1918, the average flow in one of the streams measured 6.4 million gallons per day, Grumet Prouty says. Stream gauges have recorded a gradual, long-term decline in that flow, however, and by 2004 the average daily flow measured just 3.5 million gallons — a decrease of about 45 percent.

The researchers find that a similar trend in the yttrium-to-calcium ratio is seen in the corals growing offshore: From 1920 to 2005, the yttrium-to-calcium ratio chronicled in the corals at one site where significant groundwater seepage occurs dropped 38 percent. Comparable trends are recorded in corals collected at two other sites nearby.

The findings suggest that corals that grew in the area long ago — and whose skeletons remain on the seafloor there today — could serve as ancient rain gauges, for times when rainfall data is unavailable.

An understanding of how precipitation has varied in the region in the past is vital because nearly all of the island’s drinking water, as well as the water used for agricultural purposes, comes from groundwater.

First a meteorite, then a tsunami in what is now New York City

An odd layer of material found in bodies of water from Long Island Sound to the Hudson River indicates that what is now New York City suffered a tsunami about 2,300 years ago. If that weren’t disturbing enough, evidence in the sediments hints that the massive wave was the result of an extraterrestrial object striking somewhere offshore.

Sediment samples drilled from the Hudson River at several sites between 10 and 20 kilometers upstream of Manhattan contain a distinct layer that ranges from four to 50 centimeters thick. Carbon dating of wood and other organic debris in the layer indicates that the material was deposited about 2,300 years ago, says Dallas Abbott, a geologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y.

Similarities between layers of sediment in Long Island Sound and layers on the continental shelf more than 100 kilometers offshore are a sign that the event that deposited the layers was large scale, Abbott notes.

It’s highly unlikely that an undersea earthquake triggered the tsunami that washed the debris up the Hudson because no large quake-generating faults or subduction zones lie nearby.

Instead, Abbott and her colleagues say, the debris layers in the Hudson River sediments contain several hallmarks of an extraterrestrial impact. Some samples contain millimeter-sized glass spherules, which likely condensed from vaporized material spewed into the air when an object fell from space into the Atlantic Ocean nearby. Other samples contain bits of a mineral called ilmenite, and those bits show distinct signs of being subjected to the intense pressures generated by an extraterrestrial impact.

The clincher, Abbott notes, is the presence in some samples of nanodiamonds, which are almost exclusively related to extraterrestrial impacts.

Park turfgrass may hurt climate more than it helps

The lush green turf in well-maintained parks can absorb carbon dioxide from the air as it grows and store carbon in the underlying soil, thereby helping to reduce the amount of the gas building up in the atmosphere. This buildup is one cause of detrimental climate change.

Yet new analyses of greenhouse gas emissions from the soil in such parks hint that the parks may still contribute to global warming.

Amy Townsend-Small, a biogeochemist at the University of California, Irvine, and her colleagues studied the greenhouse gas emissions from various patches of turf at four parks in Irvine. All the parks had been established between 1975 and 2006 on sites formerly occupied by open grassland. Also, all of the parks’ leisure areas, such as those used for open lawns and picnic sites, are watered and fertilized according to the same procedures, Townsend-Small notes.

At the Las Lomas Community Park, which was established in 2005, the top 5 centimeters of soil beneath each square meter of turf contains about 600 grams of carbon. The same of amount of soil beneath the turf in the Harvard Community Athletic Park — which opened in 1975 and is the oldest of the group the team studied — contains about 2.6 kilograms of carbon. The researchers found that, in general, the older the park was, the more carbon it stored beneath its turf.

The field tests also indicated, however, that soils beneath turf in the older parks emitted higher amounts of nitrous oxide (N2O) than the grassy areas of the younger parks. That gas, while emitted in small quantities, is a greenhouse gas whose long-term planet-warming effect is about 300 times that of carbon dioxide. In the oldest park, in fact, the harmful effect of emitted N2O eclipsed any gain from stored carbon.

Tests conducted so far don’t pin down the cause of the N2O emissions. However, Townsend-Small and her colleagues suggest that fertilizer may be building up in the soils in parks as they age, and soil bacteria may be converting some of those accumulated nutrients into the greenhouse gas.

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