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Coral Cores For Reef Growth And Climate Change Assessment
Posted by Daria Siciliano, Ph.D. candidate, University of California, Santa Cruz

Daria Siciliano and Jim Maragos, PhD coring a large Porites evermanni coral colony.Understanding patterns of calcium carbonate production over large scales in marginal, subtropical reef environments is critical to coral reef science in many respects. First, understanding carbonate production helps assess the influence of global processes, such as rising sea surface temperatures (SST), and atmospheric CO2, on the health of coral reefs. Secondly, it helps understand the controls on reef growth and distribution worldwide. Lastly, it sheds new light on how reefs respond to the changing global environment.

Kure Atoll in the NWHI, is the world's northernmost atoll. At 28.26º N, it occurs in an ecologically marginal environment with respect to SST, which often dips below 18º C in the winter. This temperature is the lower physiological limit for corals to thrive and reefs to grow. Kure's unique position and isolation offer an ideal opportunity to investigate not only worldwide controls on reef development and distribution, but also to study the responses of coral reefs to global climate change in the absence of confounding effects from direct anthropogenic (human) activities. Grigg (1982) postulated that Kure Atoll lies at the "Darwin Point" for reef development, a geographical limit beyond which corals and coralline algae, the main players in reef accretion, hindered by low SST, can no longer deposit enough CaCO3 to compensate for subsidence of the volcanic basement. Reefs at latitudes higher than the Darwin Point will fail to remain at sea level and sink below the photic zone within which adequate calcification must occur.

I am integrating remote sensing data (IKONOS satellite imagery, among other data) and recent field surveys (NOWRAMP 2000-2002) to model carbonate accretion and reef growth at Kure Atoll and neighboring reefs in the NWHI, and test Grigg's hypothesis, focusing on patterns of benthic diversity and variations in carbonate production. Benthic habitat maps from satellite data allow habitat-specific calculations for a CaCO3 budget at the scale of the entire atoll ecosystem. Extensive in-water field surveys insure the collection of accurate data, such as coral cores, on coral and coralline algal growth rates, as well as bioerosion rates, and verification of habitat maps.

I am lucky to be working with invertebrate specialists and fish biologists here aboard the Rapture, who are independently collecting data on rock-boring urchins and fish abundance. Rock boring urchins are in fact the number one culprit for removal of CaCO3 from the reef framework. Their bioerosion activity can be quantified by obtaining data on their abundance at Kure Atoll from the invertebrate specialists. Parrotfishes are the second most important bioeroders on any reef. Their rate of CaCO3 removal can be calculated from the abundance data obtained from the fish biologists on this expedition. This is truly a multi-disciplinary, collaborative research effort!

My analyses of high resolution IKONOS satellite imagery aim at mapping the total aerial cover and spatial distribution of the reef framework builders, which in the NWHI comprise not only live coral but coralline algae as well. I am extracting coral cores from large massive Porites evermanni corals (major framework builders) at Kure and neighboring atolls and islands in the NWHI to provide historical records of coral growth rates, and to compare growth rates across the archipelago. Growth rates of other framework builders will be estimated from coral and coralline algal samples being collected throughout the NWHI. Coral density banding also reflects the possible response to changing conditions (rising SST, CO2) of this century. Ongoing analyses of these data suggest that coral and coralline algal cover at Kure atoll is higher than previously suggested, and that coral growth rates in certain habitats are an order of magnitude higher than those previously reported.

In 2001, we started collecting coral cores at French Frigate Shoals and several other NWHI to determine latitudinal gradients in coral growth rates. Because of seasonal and annual variations in skeletal density and isotopic composition, the cores can: a) be used to estimate the age and historical growth rate of large (and possibly very old) corals; and b) serve as records of seawater temperatures over several decades, and of coral growth responses to such temperatures using oxygen isotope techniques. During NOWRAMP 2002 aboard the research vessel Rapture, an additional 16 coral cores have been collected.

Daria Siciliano drilling a coral core in Porites evermanni. To collect a coral core, two divers scout a particular area looking for a suitable large massive coral in the species Porites evermanni, and if found, return to the small boat. The equipment and tools are set up in preparation for the actual coring. The core is taken using a 1/2 inch pneumatic drill equipped with a 50 cm long hollow steel cutting cylinder, that we built at UCSC. The pneumatic drill with the corer is hooked up to a SCUBA tank and carried underwater to the selected coral head. After the core is removed, it is labeled, placed in a PVC cylinder, and stored. A beveled concrete plug is then tapped into the core hole using a rubber mallet to seal off the hole, promote coral tissue regeneration, and avoid bioerosion. The coral and plugged core hole is also photographed. Depth, species, dimensions of the coral head, and date are recorded along with a GPS coordinates from the boat.

Once back at the lab at UCSC, 5mm-thick sections are cut from the cores using a rock saw along the major growth axis of each coral core. I then X-radiograph each slice to visualize seasonal and annual growth bands. Corals in fact lay down seasonal 'growth rings' much like trees. These are measured to determine skeletal growth rates. Age of cored corals can then be estimated by dividing the average radius of the head by the average annual growth increment. The cores and coral samples will also be analyzed using a mass spectrometer to determine oxygen stable-isotope (18O/16O) ratios, from which calcification temperatures can be determined. A radiocarbon (14C) analysis will also be performed on the longer cores with the expectation of observing the 14C spike of the 1960s from atmospheric bomb tests.

This research will provide new insights on how global climatic changes (e.g. increased sea surface temperature) of the last few decades have affected the location of the Darwin Point of reef development. This latitude, by virtue of its definition, should be highly sensitive to small variations in the environmental parameters conducive to reef growth, and may have shifted with respect to Kure's location.


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