Glacial‐interglacial CO₂ change: The Iron Hypothesis

TLDR

Glacial–interglacial CO₂ variations (≈200–280 ppm) are attributed to changes in southern‑ocean phytoplankton productivity and nutrient use, with dust‑derived iron supply being ~50 × higher during the last glacial maximum. The authors hypothesize that iron limitation in the modern southern ocean restricts phytoplankton from exploiting excess surface nitrate and phosphate, thereby capping new production at ~2–3 × 10¹⁵ g C yr⁻¹. Iron limitation explains why Holocene interglacial CO₂ remained ~280 ppm, while iron enrichment during the last glacial maximum likely enhanced new productivity and helped draw CO₂ below 200 ppm.

Abstract

Several explanations for the 200 to 280 ppm glacial/interglacial change in atmospheric CO 2 concentrations deal with variations in southern ocean phytoplankton productivity and the related use or nonuse of major plant nutrients. An hypothesis is presented herein in which arguments are made that new productivity in today's southern ocean (7.4 × 10 13 g yr −1 ) is limited by iron deficiency, and hence the phytoplankton are unable to take advantage of the excess surface nitrate/phosphate that, if used, could result in total southern ocean new production of 2−3 × 10 15 g C yr −1 . As a consequence of Fe‐limited new productivity, Holocene interglacial CO 2 levels (preindustrial) are as high as they were during the last interglacial (≈ 280 ppm). In contrast, atmospheric dust Fe supplies were 50 times higher during the last glacial maximum (LGM). Because of this Fe enrichment, phytoplankton growth may have been greatly enhanced, larger amounts of upwelled nutrients may have been used, and the resulting stimulation of new productivity may have contributed to the LGM drawdown of atmospheric CO 2 to levels of less than 200 ppm. Background information and arguments in support of this hypothesis are presented.