Iron fertilization enhanced coccolithophore growth rate in the northwestern Arabian Sea during the Last Glacial Maximum

The Arabian Sea is among the most productive ocean basins globally, driven by summer coastal upwelling, winter convective mixing, and aeolian dust inputs that supply nutrients to the euphotic zone. In several aspects, this region shares characteristics with High Nutrient–Low Chlorophyll (HNLC) systems, where mineral dust deposition partially alleviates iron limitation of surface waters, that are ventilated by iron-depleted waters (e.g., the Antarctic Intermediate Waters). Paleorecords indicate that enhanced dust fluxes during the Last Glacial Maximum (LGM) coincided with increased primary productivity in the northwestern Arabian Sea, suggesting a potential role for iron fertilization, although the underlying mechanisms remain poorly constrained.

Here, we reconstruct millennial-scale variations in coccolithophore growth rates in the northwestern Arabian Sea since the LGM, based on the coccolith carbon isotope vital effect (δ¹³C_VE) recorded in sediment core MD00-2354 (61.48°E, 21.04°N). Combined with coccolithophore cell-size estimates at the studied site, and reconstructed iron fluxes in the area, these data allow us to investigate the links between iron availability and phytoplankton growth from 22 to 4 ka.

Our results show that coccolithophore growth rates and cell sizes were significantly increased during the LGM, coincident with maxima in mineral dust and iron fluxes. This pattern suggests that nutrient availability was the primary control on coccolithophore growth at that time. This interpretation is supported by a positive correlation between coccolithophore growth rates and independently reconstructed net primary productivity at the site. A likely mechanism is that increased iron supply during the LGM enhanced phytoplankton nitrogen assimilation, as further supported by ROMS–PISCES model simulations. Comparisons between simulations with and without atmospheric iron deposition indicate that, under increased iron input, the enhancement in nitrogen utilization exceeds that of phosphorus utilization, and is concomitant to elevated primary productivity.