Q: How has past and current geology, historic ocean
currents and other factors influenced the diversity of the GOM? (Matthew)
A: First, lets discuss the geology of the Gulf of
Mexico. The Gulf is considered a passive
margin, because its not at the site of an active fault or tectonically active
rift, subduction zone or volcanic feature.
It is also dominated by the enormous Mississippi River flowing into
it. The continental shelf varies in
length around the Gulf – it extends the farthest around Florida (the Florida
Keys are all on the shelf). The basin
itself is also relatively shallow, compared to deep ocean basins. These features all affect the type of
sediment and substrate found at the bottom of the Gulf of Mexico – mostly reefs
(around Florida) and soft-bottom substrate.
Also, along the northern Gulf continental slope (that’s the steep part
between the shelf and the basin, usually about 200 – 3000 m water depth), there
are extensive salt domes and carbon-rich deposits embedded in the slope, shelf
and continental rise. This is where all
the oil and gas drilling and mining occurs.
Oil and gas deposits suggest that parts of the Gulf of Mexico were
filled with carbon-rich deposits (such as plants and animals in abundance living,
dying, and becoming deposited on the substrate and subsequently buried). Such a situation could have occurred with low
stands of the sea, where sea level was much lower than it is now (up to 200 m
lower), and rivers emptied directly onto the continental shelf. In such a low stand of the sea, organic
deposits could build up and eventually become fossilized or sedimented. When sea level rose again, the shelf would be
covered with water and marine sedimentation would begin again, and thus we see
layers of carbon rich deposits and marine sediments.
The canyons of the slope and rise are built by turbidity
currents. Turbidity currents are gravity
flows of sediment, mobilized with water and remain in motion because of the
turbidity. Turbidity currents move
because the sediment is much more dense than water, and so the turbidity of a
dense material creates a flow that accumulates as it “runs”, and turbidity
currents can blast through the topography at extraordinary speeds. Turbidity currents leave “turbidites” behind
in their wake, or sediments from turbidity currents. These turbidites resemble riverine delta
regions in terms of deposition grain size – the large grains settle out first,
then silts and clays last. Turbidity
currents and turbidites are important features of the GoM.
As for biodiversity in the Gulf of Mexico, there are
connections with the geology and physical oceanography of the basin, as these
are often the driving factors behind diversity.
However, also playing a role in biological production are the
availability of nutrients and appropriate habitats for each trophic level. The benthic organisms are the most diverse
within the marine species, so if there are good habitats and growth conditions
for benthics then there will be high diversity. For this reason, some have
chosen to measure substrate diversity as a proxy for biodiversity of the
system. In reality, there are more
factors than this contributing, especially including nutrients and recruitment
ability of each population. However, the
Gulf plays host to some of the most diverse benthic ecosystems on the
planet: coral reefs. Reef systems tend to have the most diversity
of any marine ecosystem type, the “tropical rainforest” of the sea. Since the Gulf of Mexico does have some coral
reefs, especially in the shelf area off Florida, it can be considered to have
relatively high diversity. Also, as the
Gulf of Mexico is NOT a big deep basin with unproductive oceanic gyres in it,
the whole of the basin is relatively productive. This doesn’t necessarily mean its more
diverse, but more productivity is a good starting point for being able to have
high diversity. There are smaller
mesoscale eddies that transport plankton, nutrients and fish larvae around the
basin, along with currents, but there are no upwelling or downwelling features
that impact biological productivity on a large scale. The “global conveyor belt” of thermohaline
circulation is such that the Gulf receives surface (warmer, less nutrients)
water from the Atlantic, but probably none of the North Atlantic Deep Water or
nutrient and oxygen-rich deep waters. Biological
production is limited to the nutrients that are provided via the rivers,
Aeolian input and from marine sediments.
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