Equatorial Pacific dust fertilization and source weathering influences on Eocene to Miocene global CO₂ decline

Eolian dust, carrying iron to the oceans, plays a crucial role in atmospheric CO₂ removal through enhanced primary productivity and subsequent carbon sequestration. The eastern equatorial Pacific (EEP) Ocean, an iron-limited high-nutrient, low-chlorophyll (HNLC) region, is thought to be a key player in this process. However, the extent of dust fertilization's influence on long-term CO₂ changes, particularly during major climate transitions, remains debated. Some studies suggest ocean dynamics are more important than dust input in controlling EEP productivity, while others question the significance of EEP iron fertilization over certain time periods. This study aims to clarify the role of dust fertilization in EEP CO₂ sequestration during the Eocene-Oligocene Transition (EOT), a period marked by significant global cooling and the establishment of the Antarctic ice sheet. Specifically, it addresses three key questions: (1) the dominant iron sources limiting EEP productivity; (2) whether the EEP has always been iron-limited; and (3) whether iron inputs, especially from eolian dust, regulated atmospheric pCO₂. To achieve this, the study utilizes samples from Integrated Ocean Discovery Program (IODP) Site U1333, located in the EEP HNLC region. Magnetofossils (remains of magnetotactic bacteria) are used as a paleoproductivity indicator, along with chemical weathering information from eolian dust to reconstruct dust source region weathering intensity.

Previous research on the relationship between iron fertilization and the carbon cycle has yielded conflicting results. While studies like Kolber et al. (1994) and Coale et al. (1996) highlight the importance of iron limitation and the effectiveness of iron fertilization experiments in stimulating phytoplankton blooms, other works (Winckler et al., 2016; Ziegler et al., 2008; Costa et al., 2016) challenge the prevailing narrative by suggesting that ocean dynamics or a lack of significant fertilization events during certain periods outweigh the impact of dust inputs. These conflicting findings highlight the need for a more comprehensive understanding of the interplay between dust fertilization, ocean circulation, and long-term carbon cycle dynamics. The use of traditional proxies like opal and total organic carbon (TOC) has also faced challenges due to preservation issues, prompting the search for more robust indicators, such as magnetofossils, as explored in this study.

The study analyzed samples from IODP Site U1333, focusing on a ~170-m thick sediment sequence. Magnetofossils were identified and quantified using isothermal remanent magnetizations (IRMs), first-order reversal curves (FORCs), and transmission electron microscopy (TEM). The abundance of magnetofossils, representing past biological pump activity, was determined using anhysteretic remanent magnetization (ARM_20mT) measurements, with chemical treatment to differentiate biogenic magnetite from silicate-hosted magnetic inclusions. Eolian dust input was assessed through 'operationally defined eolian dust' (ODED) analysis, involving sequential chemical extraction and geochemical analysis (La-Th-Sc diagrams) to minimize contamination from volcanic ash. The chemical index of alteration (CIA) was calculated from geochemical data to infer the intensity of chemical weathering in the dust source region. The study also used total Fe, opal, and TOC measurements in the bulk sediment samples to further understand the relationship between iron supply, productivity, and carbon burial. Nd isotopic ratios (εNd) were measured to determine the provenance of the eolian dust. The relationship between various proxies (including magnetofossils, dust flux, CIA, and atmospheric CO2) were analyzed and correlated to understand the feedback mechanisms during the EOT. Flux calculations were made using mass accumulation rates (MAR), with corrections made for carbonate compensation depth changes. The age model used was based on the timescale of Westerhold et al. (2012).

The study successfully employed magnetofossils as a reliable proxy for past EEP dust fertilization and biological pump activity. The results demonstrate a strong correlation between eolian dust input (ODED, HIRM_300mT) and sedimentary iron content, indicating that dust was the dominant external iron source to the EEP from the late Eocene to early Miocene. Magnetofossil abundance (ARM_20mT) shows a better correlation with iron than traditional proxies like opal and TOC, which are prone to preservation issues. Analysis of LaN/YbN ratios suggests that the EEP dust originated primarily from the Asian interior. The study further revealed that the Intertropical Convergence Zone (ITCZ) was located south of the equator during the Eocene. A significant finding was the contrasting chemical weathering intensities (CIA) across the EOT, indicating a shift from weaker weathering before the EOT to significantly stronger weathering afterward. This enhanced chemical weathering in the Asian dust source region, potentially linked to intermittent northern hemisphere cooling and increased moisture, contributed to higher atmospheric CO₂ removal. However, a decline in EEP dust input around the EOT weakened the dust-fueled biological pump, moderating the global CO₂ decline. The study also found that the variations in magnetofossils, dust, CIA, and atmospheric CO2 levels showed strong correlations over time, indicating intricate links between Asian interior conditions, EEP dust fertilization, and global climate change.

The study's findings address the long-standing debate on the role of dust fertilization in the EEP by providing robust evidence for its significant impact on the long-term carbon cycle, particularly across the EOT. The use of magnetofossils as a proxy for biological pump activity overcomes limitations associated with traditional proxies. The correlation between dust input, chemical weathering intensity, and atmospheric CO2 levels reveals a complex interplay of factors influencing global climate. The shift in chemical weathering patterns suggests that continental weathering played a crucial role in enhancing CO2 drawdown, but weakening of the EEP dust fertilization provided a moderating feedback. This moderating effect is hypothesized to have counteracted the substantial CO2 drawdown caused by intensified continental weathering, illustrating a complex interplay of carbon cycle feedbacks during major climate transitions. These findings are crucial in improving our understanding of the various mechanisms that contributed to the major climate shift at the EOT.

This research demonstrates the effectiveness of magnetofossils as a valuable proxy for tracing past dust fertilization and its influence on carbon burial. The study reveals a complex interplay between Asian interior environmental conditions, EEP dust fertilization, and global CO₂ decline across the EOT. Intensified chemical weathering in the Asian source region enhanced CO₂ removal, while decreasing EEP dust fertilization moderated this decline. Future research could explore the precise mechanisms linking northern hemisphere cooling to changes in moisture availability and weathering intensity in the Asian interior and improve the accuracy of proxy data in older sediments.

The study focuses on a single IODP site. While the Nd isotopic data suggest a consistent Asian dust source, incorporating data from additional sites could enhance the regional and global applicability of the findings. The interpretation of CIA values relies on certain assumptions regarding provenance and diagenesis; further investigation to refine these aspects would strengthen the study's conclusions. The impact of potential volcanic ash contamination, though assessed, may still pose some uncertainty. Improved methods to completely eliminate this impact would increase confidence in the results.