The Biological Pump

Despite having residence times (τ) that exceed the ˜1,000yr mixing time of the ocean (Broecker and Peng, 1982), many dissolved constituents of seawater have distributions that vary with depth and from place to place. For instance, silicic acid (τ=1.5×104 yr), nitrate (τ=3,000 yr), phosphate (τ=(1-5)×104 yr), and dissolved inorganic carbon (DIC; τ=8.3×104 yr) are generally present in low concentrations in surface waters and at much higher concentrations below the thermocline (Figure 1). Additionally, their concentrations are higher in older deep waters than they are in the younger waters of the deep sea (Figure 2). This is the general distribution exhibited by elements and compounds taking part in biological processes in the ocean and is generally referred to as a "nutrient-type" distribution. (16K)Figure 1. Depth profiles of: (a) ∑CO2, (b) dissolved CO2, (c) silicic acid, (d) nitrate, and (e) phosphate from the Indian Ocean (27° 4' S, 56° 58' E; GEOSECS Station 427) (source Weiss et al., 1983). (22K)Figure 2. Nitrate concentrations along the great ocean conveyor at 2,000 m depth (source Levitus et al., 1994, by way of the LDEO/IRI Data Library). Both the lateral and vertical gradients in the concentrations of nutrients result from "the biological pump" (Figure 3). Dissolved inorganic materials (e.g., CO2, NO3-, PO43-, Si(OH)4) are fixed into particulate organic matter (carbohydrates, lipids, proteins) and biominerals (silica and calcium carbonate) by phytoplankton in surface waters. Some of these particles are subsequently transported, by sinking, into the deep. The bulk of the organic material and biominerals decomposes in the upper ocean via dissolution, zooplankton grazing, and microbial hydrolysis, but a significant supply of material does survive to reach the deep sea and sediments. Thus, just as biological uptake removes certain dissolved inorganic materials in surface waters, the decomposition of sinking biogenic particles provides a source of dissolved inorganic material to deeper waters. Thus, deeper waters contain higher concentrations of biologically utilized materials than do surface waters. Older deeper waters contain higher concentrations of bums compared to newly formed deep waters or surface waters. (7K)Figure 3. Diagram of the biological pump (after OCTET workshop report). One side-effect of the biological pump is that CO2 is shunted from the surface ocean and into the deep sea, thus lowering the amount in the atmosphere. For many years it has been recognized that pre-Industrial CO2 levels in the atmosphere were about one-third of what they would be in the absence of a biological pump (Broecker, 1982). It is also known that the biological pump is not operating at its full capacity. In so-called "high-nutrient, low-chlorophyll" (HNLC) areas of the ocean, a considerable portion of the nutrients supplied to the surface waters is not utilized in support of primary production, most likely due to the limitation of phytoplankton growth by an inadequate supply of trace elements (e.g., Martin and Fitzwater, 1988). The possibility that the biological pump in HNLC areas might be stimulated by massive additions of iron both artificially as a means of removing anthropogenic CO2 from the atmosphere and naturally as a cause for the lower glacial atmospheric CO2 levels (Martin, 1990) is the focus of much research and debate (e.g., Martin et al., 1994; Coale et al., 1996; Boyd et al., 2000; Watson et al., 2000).Although the biological pump is most popularly known for its impact on the cycling of carbon and major nutrients, it also has profound impacts on the geochemistry of many other elements and compounds. The biological pump heavily influences the cycling, concentrations, and residence times of trace elements - such as cadmium, germanium, zinc, nickel, iron, arsenic, selenium - through their incorporation into organic matter and biominerals (Bruland, 1980; Azam and Volcani, 1981; Elderfield and Rickaby, 2000). Scavenging by sinking biogenic particles and precipitation of materials in the microenvironment of organic aggregates and fecal pellets plays a large role in the marine geochemistry of elements such as barium, thorium, protactinium, beryllium, rare earth elements (REEs), and yttrium ( Dehairs et al., 1980; Anderson et al., 1990; Buesseler et al., 1992; Kumar et al., 1993; Zhang and Nozaki, 1996). Even major elements in seawater such as Ca2+ and Sr2+ display slight surface depletions (Broecker and Peng, 1982; de Villiers, 1999) as a result of the biological pump, despite their long respective oceanic residence times of 1 Myr and 5 Myr (Broecker and Peng, 1982; Elderfield and Schultz, 1996)