Each year, in the spring and again in the fall, SoMAS professors Mary Scranton and Gordon Taylor head for the Caribbean. But their intention is to work, not to play. Since 1995, Scranton and Taylor have been principal investigators on a bi-national, multi-institutional study of how large-scale oceanic-atmospheric phenomena and changes in climate affect the vertical flux of carbon and nutrients at a continental shelf site. That site is the Cariaco Basin on the north-central coast of Venezuela.
The Cariaco Basin is comprised of two deep east-west trending basins separated by a relatively shallow (900m) sill. To the north, shallow sills restrict the exchange of water between the basins and the Caribbean Sea. High rates of primary productivity in surface waters and reduced circulation within the basin result in the waters below about 250m depth being permanently anoxic. Thus, the sediments in the basins are unaffected by bioturbation and their vertical stratigraphy reflects changes in primary productivity in overlying waters associated with the region’s strong seasonal upwelling cycle. Basically, the Cariaco Basin is one very large sediment trap. What happens in the Basin stays in the Basin!
The Cariaco Basin’s unusual geography and undisturbed sediments make it an excellent place to study tropical climate change. Because its waters vertically grade from well oxygenated at the surface to anoxic below 250m, it is also ideal for the study of how organic materials decompose along an oxic-anoxic gradient. The examination of both these phenomena are among the objectives of the Carbon Retention in a Colored Ocean (CARIACO) study, a long-term, time series study of the basin launched in 1995 by the National Science Foundation and the Fondo National de Ciencia, Technologia e Innovacion, the comparable Venezuela federal agency. The CARIACO project involves teams of scientists from several US and Venezuelan universities.
Scranton and Taylor study the geochemistry and the microbiology of the water column in the Basin with the essential support of a Venezuelan team, who recently completed its 175th scientific cruise.
“Mary‘s and my interests are really about the geochemistry and the microbiology that occurs across the transitional waters from where there is oxygen to where there’s no oxygen,” says Taylor. “That’s where all the interesting changes are that make this particular part of the ocean different than most other regions… From a microbiologist’s perspective, it’s interesting because there are lots of novel types of metabolism.”
The Basin’s anoxic depths may offer a clearer view into the dynamics of the ancient ocean, which was largely anoxic.
“Studying modern systems gives us some perspective on the past, on the evolution of marine systems from the totally anoxic to the way they are today where there are just pockets of anoxia in restricted areas. It gives us some idea of the historical development in the geochemistry of the ocean as well as a perspective on the evolution of marine microorganisms,” said Taylor.
About 2.8 billion years ago, the introduction of cyanobacteria caused the gradual oxidation of the ancient ocean.
“But, even after the great oxidation event, climatic variations and changes in ocean circulation led to the ocean oscillating between oxic and anoxic conditions. There were periods in the distant past, millions of years ago, where the vast majority of the ocean was anoxic again, so it has moved in and out of anoxia,” said Taylor.
The CARIACO program includes two other projects coordinated by the University of South Florida and the University of South Carolina. The other projects are focused on remote sensing in global carbon fluxes from the point of view of the other times series sites and the long-term climate record based upon the sediment record, respectively.
Because the Cariaco Basin system is variable and sensitive to climatic forcing, the study has benefited from its long duration.
“What we have learned in oceanography over the past 30 years is that, in fact, things fluctuate a lot and you can’t really tell what is a long-term trend and what is a short-term variation,” said Scranton, who has been studying the Cariaco Basin since the 1970s. “We’re looking at what’s happening on an annual basis and hopefully the geologists will be able to connect that with what is being preserved in the sediment on an annual basis …then you’re not just guessing what the sediment signal means…you have some real-time connection with what’s going on at the surface.”
Taylor said that time series observations allow patterns to become evident.
“The advantage of the time series is you get to make the same measurements again and again at the same place and start to actually see temporal patterns, time-varying patterns that you could only guess at by historical oceanographic sampling approaches, which equate to snapshots in space and time” said Taylor. “To my way of thinking, asking what the value of a time series is in the ocean, is like asking ‘why bother having a weather station on land to monitor rainfall, temperature?’.”
Taylor said people should be concerned because of the increasing pressure society is putting on coastal environments. The human addition of excess nutrients stimulates the production of plankton and ultimately leads to the creation of anoxic “dead zones”, as seen seasonally in the Gulf of Mexico and Long Island Sound.
“I think we understand pretty well the causes of it but the long-term effects are not well understood,” said Taylor. “Studying anoxic systems will help us better predict what’s going on there.”
From a geologic standpoint, an anoxic system allows for the preservation of sediments, which allows for the investigation into the ancient climate.
“Part of the motivation for our project is to document contemporary processes and how they’re preserved in surface contemporary sediments and to use that information to better interpret the ancient climate record. Better understanding of the past leads to a better prediction of the future,” said Taylor.
Scranton and Taylor’s colleague, SoMAS Professor David Black, is recreating the tropical climate during the Holocene geologic period (± last 10,000 years) using microfossils and their isotope signatures collected from the Cariaco Basin.
“In terms of tying in our work with what’s going on with the sediments, what goes on in the water column is going to influence what is preserved in the sediments so the geologists tend to only think about the surface and the bottom and they don’t worry about the processing…we’re trying to either prove that it is insignificant or show them what’s changing, what they do need to worry about,” said Scranton.
In their field work in the Basin, Taylor and Scranton utilize a SeaBird rosette, which contains a number of instruments that measure water temperature, salinity, water clarity, acidity and the fluorescence of plankton. The rosette also has 12 bottles which each capture 8 liters of water at specific depths.
There is a reason why Scranton and Taylor go to Venezuela twice a year, once in the spring and once in the fall.
“This is a system that has five months that are highly productive due to upwelling. The upwelling season starts about [December] and runs through May; we always try to capture samples during upwelling…when productivity is near its peak and then again in the fall when productivity is at is minimum,” said Taylor. “So every year this gives us the largest contrast in our measurements.”
Taylor observes that, during the 15-year course of the time series, the pH of the Basin’s water has decreased, primary productivity has decreased and the relaxation of the trade winds has resulted in decreased upwelling, which contributes to the nutritional sustenance of phytoplankton. The phytoplankton community composition changed dramatically in 2005 to smaller species. Furthermore, during the study’s duration, the water’s surface temperature has increased by 1o C.
“Scientists recognize that, without time series measurements, we really can’t talk authoritatively about the effects of climate change on the ocean,” said Taylor.