Abstract
Although iron (Fe) is a key regulator of primary production over much of the ocean, many components of the marine iron cycle are poorly constrained, which undermines our understanding of climate change impacts. In recent years, a growing number of studies (often part of GEOTRACES) have used Fe isotopic signatures (δ56Fe) to disentangle different aspects of the marine Fe cycle. Characteristic δ56Fe endmembers of external sources and assumed isotopic fractionation during biological Fe uptake or recycling have been used to estimate relative source contributions and investigate internal transformations, respectively. However, different external sources and fractionation processes often overlap and act simultaneously, complicating the interpretation of oceanic Fe isotope observations. Here we investigate the driving forces behind the marine dissolved Fe isotopic signature (δ56Fediss) distribution by incorporating Fe isotopes into the global ocean biogeochemical model PISCES. We find that distinct external source endmembers acting alongside fractionation during organic complexation and phytoplankton uptake are required to reproduce δ56Fediss observations along GEOTRACES transects. δ56Fediss distributions through the water column result from regional imbalances of remineralization and abiotic removal processes. They modify δ56Fediss directly and transfer surface ocean signals to the interior with opposing effects. Although attributing crustal compositions to sedimentary Fe sources in regions with low organic carbon fluxes improves our isotope model, δ56Fediss signals from hydrothermal or sediment sources cannot be reproduced accurately by simply adjusting δ56Fe endmember values. This highlights that additional processes must govern the exchange and/or speciation of Fe supplied by these sources to the ocean.
Highlights
The Asian summer monsoon (ASM) sustains roughly 60% of the global population (Li et al, 2016) and serves as the main moisture supply for Asia (Webster et al, 1998)
The results show that the northern edge of the ASM migrated northwestward by ∼200 km, ∼50 km, and ∼50 km with global warming from the LGM to preindustrial, from the preindustrial to mid-Holocene, and from the mid-Holocene to mid-Pliocene, respectively
The summer surface air temperature (SAT) in the Eurasia mainland increased by ∼1.5°C during the mid-Holocene (Figure 1c), while the sea surface temperatures (SSTs) of the South China Sea and the equatorial western Pacific cooled by ∼0.5°C (Figure 1d)
Summary
The Asian summer monsoon (ASM) sustains roughly 60% of the global population (Li et al, 2016) and serves as the main moisture supply for Asia (Webster et al, 1998). Meteorologists have shown that changes in the intensity of the ASM are reflected by the advance and retreat of the monsoon frontal rain belt. It is generally accepted that the more northerly the penetration of the frontal rain belt, the greater the intensity of the summer monsoon (Tao & Chen, 1987). The northern edge of the ASM, defined as the northern limit of the monsoon precipitation (Chen et al, 2018; Hu & Qian, 2007) and geographically parallel to the wet– dry transitional area (Qian et al, 2009), delineates the advance and retreat of the summer monsoon rain belt (Lan et al, 2020; Yang et al, 2015). The Last Glacial Maximum (LGM, ∼21,000 yr BP), the preindustrial, the mid-Holocene (∼6,000 yr BP), and the mid-Pliocene (∼3.0–3.3 Ma) have been widely studied
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