Biogeochemical Impacts of Dust on the Global Carbon Cycle - Mineral Dust: A Key Player in the Earth System - page 364

14.3 Biogeochemical Impacts of Dust Deposition

on the Oceans

The differences between the oceans and the land surface profoundly alter the
controls on primary production and the impacts of dust on both. We therefore begin
this section with a short introduction to the broad patterns of ocean physics and
biogeochemistry based on recent reviews (Boyd and Ellwood 2010 ; Chester and
Jickells 2012 ; Moore et al. 2013 ) in order to provide a context within which to
discuss the impacts of dust on this system.
Primary production is dominated by microscopic phytoplankton and is confined
to the upper approximately 100 m (called the euphotic zone) of the oceans whose
average depth overall is about 4 km. Ocean waters are almost exclusively well
oxygenated and have a pH of about 8. In tropical waters surface heating produces
a stable less dense surface mixed layer within which photosynthesis can take place,
but losses of nutrients associated with sinking phytoplankton cells lead to very
low nutrient concentrations, and rates of primary production controlled by rates of
recycling within the euphotic zone, and rather inefficient and poorly understood
inputs from below. Decomposition of sinking material at depth leads to increases
in nutrient concentrations at depths below a few hundred metres in the water
column. In temperate regions, cooling in winter reduces surface water density and
allows mixing of the water column depths of many hundreds of metres below the
surface, which effectively returns nutrients to the sunlit surface waters. This leads to
relatively high surface water nutrient concentrations, but the low winter light levels
limit productivity. As spring arrives, heating and reduced winds allow water column
stratification, and this along with improved light levels allows a major seasonal burst
of growth (the spring bloom) followed by a summer season of nutrient depletion
with growth limited by nutrient recycling within the euphotic zone or supply from
below, before winter mixing resets the seasonal system. Underlying this surface
seasonal cycle is the massive global thermohaline circulation, which carries water
at abyssal depths from the North Atlantic through the Pacific and Indian Oceans to
ultimately return to the North Atlantic via a shallower circulation on timescales
of a few thousand years. Although there are gradients in mixing and primary
productivity across all the oceans, this simple description of the seasonality does
allow us to broadly separate the oceans into biogeochemical provinces (Longhurst
2007 ) defined by the physical mixing cycles, which, at the coarsest level, divides
the oceans into:

1. The warm tropical ocean gyres with low nutrients and relatively little seasonal
variation in primary production which is sustained throughout the year
2. The temperate and polar waters subject to seasonal mixing and systematic
seasonally variable primary production

Coastal waters cycle nutrients rather differently because loss to great ocean
depths cannot occur, but since these coastal areas are relatively small compared
to the open oceans and little impacted by atmospheric deposition compared to