Prospective use of molecular paleontology to test for iron limitation on marine primary productivity

An interesting hypothesis has recently been advanced that primary productivity in large expanses of the Pacific Ocean is limited by the eolian input of iron. I present organic geochemical data from a sediment core from one of these areas, the eastern tropical Pacific, as evidence in support of this hypothesis. The data show that the deposition of total organic carbon (TOC), primarily derived from marine productivity, reached its maximum at 18 ka and is highly correlated with the input of eolian dust. The data also show that the timing of the period of maximum deposition for biomarkers of prymnesiophyte productivity preceded the maximum for TOC by 4–6 kyr. The temporal offset between these two records of primary productivity, one a subset of the other, suggests that an ecological change in the phytoplankton community occurred in surface waters over the period of the last glacial transition as a consequence of systematic variation in the supply of eolian dust and associated bioavailable iron. This inference remains highly speculative, however, because a set of biomarkers to define the climatic response of other major components of the overall phytoplankton community during this transition period is as yet unidentified. A general discussion is developed identifying (I) the types of biomarkers that are needed to confirm the significance of iron limitation on primary productivity throughout the World Ocean and (2) new analytical approaches that might be employed to recover such biomarker information from the marine sedimentary record.