Report 18 -- Q6

What are the sediments telling us?

The APFZ and waters to the south comprise the largest area of modern siliceous sediment accumulation in the world, accounting for at least 50% of the global removal of Si from the ocean (e.g., Tréguer et al., 1995). This accumulation of biogenic - overwhelmingly diatom - material has been taken in the past as evidence of high primary productivity, or at least high diatom productivity, in Southern Ocean surface waters (e.g., Mortlock et al., 1991). However, as discussed above, direct measurements of 14C productivity and satellite images of ocean color combine to indicate that the average primary productivity of the Southern Ocean is < 50 gC m^-2 y^-1, lower than that of any other oceanic region except the permanently ice-covered central Arctic (e.g., Smith and Sakshaug, 1990). This paradoxical situation - rapid, globally significant accumulation of diatomaceous sediments in a region where the annual primary productivity is very nearly the lowest on Earth - leads to two questions:

What processes support the formation of major opal deposits in the Southern Ocean? and,
When does siliceous sediment accumulation indicate a period of high primary productivity and when does it instead reflect processes similar to those occurring in the modern Southern Ocean?

Studies of the cycling of siliceous and organic matter in the water column and upper sediments of the Ross Sea suggest that differential preservation (i.e. enhanced preservation of opal over organic matter) plays a major role in the formation of opal-rich, organic-poor sediments in that system (DeMaster et al., 1992). However, the Ross Sea is a relatively shallow (300 - 700 m) nearshore area whose annual primary productivity is about 140 gC m^-2 y^-1, about 3 times as high as that of the Southern Ocean as a whole (Smith et al., 1996). So the applicability of findings from the Ross Sea to the rest of the Southern Ocean is questionable. Globally, enhanced opal preservation appears to dominate over high primary productivity as the process responsible for the formation of siliceous sediments, and there is indirect evidence that enhanced preservation results from processes that are peculiar to diatom blooms (Nelson et al., 1995). If correct, and applicable to Southern Ocean deposits, this general trend would imply that the occasional, spatially restricted diatom blooms that are known to occur in the Southern Ocean in summer are the main source of the opal that is transported to the sea floor and preserved. The opal signal in modern sediments appears to be greatest in the APFZ, where surface sediments containing >95 weight % opal occur in some places (Bareille, 1991). This observation is supported by the fact that significant diatom blooms, composed of species that are abundant in the sediments, have been observed during the European JGOFS studies in the Weddell Sea (e.g., Quéguiner et al., 1995).

The organic-poor character of surface sediments in the APFZ, and throughout the Southern Ocean, indicates that those processes that deliver diatom opal to the sea floor do not also carry significant quantities of organic carbon to the sea floor. However, areas of significant opal export from the surface layer must also be areas of significant carbon export unless the decoupling of the cycles of silica and carbon is virtually complete within the surface layer. That is clearly not the case in the Ross Sea, where sediment-trap data indicate that the C/Si ratio of particles sinking through the 250 m depth horizon is 4 - 5 times as great as the C/Si ratio in the sediments, even though those sediments are at much shallower depths and have considerably greater organic carbon content than those in the APFZ (DeMaster et al., 1992). Thus, those processes delivering opal to the seabed may greatly intensify the transport of organic carbon to the deep ocean in the APFZ, even though the decomposition of that organic material within the deep water column is virtually complete.

The APFZ appears to be an area of significant net uptake of CO₂ by the ocean (e.g., Takahashi et al., 1986). To the extent that biological pumping processes contribute to that uptake, the biogenic particle export indicated by the opal sediments of the APFZ appears to be reflected in the pCO₂ signal. So even though opal sediments may be a poor indicator of total primary productivity they may be a much better indicator of organic matter export and biologically mediated CO₂ uptake by the ocean.

Implications for field measurements: It would be useful to know whether the pelagic decoupling between the cycles of silica and carbon observed in the Ross Sea pertains also to the APFZ, where the largest and most quantitatively important area of modern opal sediment accumulation in the ocean is found. Of more direct importance to the study of carbon cycling would be an assessment of the upper-ocean processes that result in enhanced transport of diatom silica to the seabed and the degree to which organic carbon is carried to the deep ocean by those same processes. On the basis of presently available data from the Ross and Weddell Seas (e.g., DeMaster et al., 1992; Quéguiner et al., 1995), bloom areas--even though spatially restricted and of minor importance in the overall annual primary productivity of the Southern Ocean--may be of great quantitative importance in the vertical export of both siliceous and organic biogenic material.

Implications for modeling: The seabed signal in the APFZ is derived from the production and vertical export of biogenic opal while the biological component of the atmosphere-to-ocean CO₂ flux is driven by the production and export of organic carbon. This difference implies that biogeochemical models of the Southern Ocean should consider the individual dynamics of carbon, nitrogen and silica. To resolve the sediment signal and its relationship to upper-ocean processes the models should also include at least two functionally different categories of phytoplankton--diatoms and smaller forms that neither take nor require Si. Moreover, such models should be formulated to address the possibility that there are strongly nonlinear relationships between production and export--at least for opal and perhaps for organic matter as well. Models with that degree of flexibility would be of great help in identifying relationships that result in enhanced export of opal and (perhaps) organic matter from surface waters during diatom blooms.