NOWRAMP
2002
Coral
Cores For Reef Growth And Climate Change Assessment
Posted by Daria Siciliano, Ph.D. candidate, University of
California, Santa Cruz
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.
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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