Earth, wind, flora sway Trinidad sulfur levels
As scientists observe the force of nature through a satellite weather tracker, they only see the day's events. To observe the long-term atmospheric influence, University of Cincinnati geologists are taking research a step further by tracking and measuring the distribution of sulfur in plants in the Caribbean island of Trinidad.
A new study out of the University of Cincinnati suggests that coastal proximity, rain and prevailing wind direction can all influence the distribution of marine sulfur on land. While science has known for decades that sulfur is a useful method for tracking diet and mobility, UC researchers say the combined effects of wind and precipitation like rain and ocean spray hasn't been fully investigated before.
Their results, also aligning closely with patterns reported for soils and precipitation in the Mediterranean and Pacific Islands, demonstrate that plants in coastal settings are utilizing marine-derived sulfur.
"What makes our study unique is that we were able to show quite clearly how the spatial distribution of sulfur isotopes in vegetation is related to not just coastal proximity, but also wind and rain," says Brooke Crowley, UC associate professor of geology and anthropology. "And we have shown that these spatial patterns are detectable in vegetation.
"This information may aid researchers in tracking not only the origin of human resources and the movement of animals but also sulfur emissions from human activities within the Caribbean."
Crowley and Janine Sparks, a UC doctoral student in geology at the time and first author on the study's publication, joined researchers at the University of the West Indies to measure sulfur isotope levels from wind-blown ocean spray on Trinidad's weedy native plants.
"We looked at the sulfur content in plants found across the island of Trinidad to see how windward coastal locations compared to inland and leeward wind coastal areas," says Crowley.
The study, published in Applied Geochemistry, described some interesting findings. Along windward coastlines with the most torrential rain and strongest winds blowing off the ocean, the chemical detectives found what they expected—the highest concentrations of marine-derived sulfur.
"But in the middle of the island, there was not as much as we thought we would see," says Sparks, now a laboratory manager in the Department of Earth, Atmospheric and Planetary Sciences at Purdue University. "Levels started dropping off between 1.5 to 10 kilometers inland.
"We based our expectations on a previous study other researchers performed on sheep's wool in Ireland. There they found a higher content of marine sulfur on sheep near the coast but almost as high a content level for sheep who lived 100 kilometers inland from the windy coast."
While sheep obvously travel more than plants, Sparks says the high content of marine-derived sulfur on inland sheep stems more from the environment than their movement. Unlike Trinidad's dense terrain, Ireland's lack of a diverse variety of plants, especially tall vegetation, and much stronger winds tend to carry the sulfur further inland in a very steady direction. This would enable the sea spray to travel farther without any windbreak.
The UC researchers' results were more similar to patterns reported for soils and precipitation in Japan and Hawaii. Like Trinidad, these islands are peppered with a lot of diverse vegetation and mountains, and marine-derived sulfur is only found up to 16 kilometers inland, "so we were not completely unique," Sparks adds.
'Reigning' sea spray
The results of this study, the researchers say, helped establish clear spatial patterns in the Carribbean because, although there have been other papers looking at sulfur isotopes in other parts of the planet, they may not be globally relevant. The researchers needed a baseline for what was going on regionally.
While focusing on wind direction and precipitation patterns across Trinidad's diverse terrain, they found differing oceanic sulfur signals between windward coasts blowing sea spray directly in from the ocean compared to coasts that are more leeward where wind is coming from across the island.
"If you travel from the east windward coast of Trinidad to the west coast there are obvious gradients. Within about 1.5 kilometers from the east coast, we see plant sulfur values that look just like marine sulfur," says Crowley. "The plants are using sulfur spray being blown in off the ocean, but there's also more rain there so we can't completely disentangle the influence of wind and rain—they work in concert.
"On the leeward west coast, we don't see that. Coastal plants resemble plants further inland."
Human activities may be affecting these patterns, specifically. Emissions from oil refineries and vehicles appear to have a measurable influence, they found.
"Sulfur isotope values of plants near busy roads or urban centers are clearly affected by modern human influences," says Sparks. "Using these results and other modern datasets for reconstructions of the past, we need to acknowledge what sources would and would not have been present."
In an associated study published prior to this one, Crowley and Sparks looked at carbon, nitrogen and sulfur isotope values in modern vegetation as well as land animal remains from three coastal archaeological sites in southwestern Trinidad.
Through a thorough understanding of the uneven distribution of sulfur isotope levels across the island, values found in vegetation can be used to track resource use, geographical origin and mobility of animals or people, says Crowley.
"After establishing what spatial variability is like for each of these isotope systems using plants, we can look at the mobility of organisms or where things are sourced," says Crowley. "For example, if the remains of a deer are recovered from a coastal location—but have a low sulfur isotope value—we know the deer must have come from further inland.
"It could have moved to its current location and died here or it may have been brought to the location by people. The important thing is that we can confirm it did not originate or live at the location where it was recovered. We can use this approach to investigate where people obtained their food in the past," she adds.
"We hope our studies will be of use for future ecological and archaeological research, not only for Trinidad but for the Greater Caribbean and other coastal or island systems around the world."
More information: Janine M. Sparks et al, Coastal proximity, orientation, and precipitation amount drive spatial variability in δ34S values on the Caribbean island of Trinidad, Applied Geochemistry (2019). DOI: 10.1016/j.apgeochem.2019.104372
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