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

The eastern equatorial Pacific (EEP) is a high-nutrient, low-chlorophyll (HNLC) region where primary productivity is widely considered to be limited by iron availability rather than grazing pressure. Iron-bearing eolian dust delivered from continents can fuel marine primary productivity and enhance the biological pump, thereby promoting long-term carbon sequestration. However, the extent to which dust fertilization has influenced EEP productivity and atmospheric CO₂, particularly across major climate transitions such as the Eocene–Oligocene Transition (EOT, ~34 Ma), remains debated. Some studies suggest ocean dynamics controlled EEP productivity over the last 0.5 Ma, and others infer limited roles for iron fertilization over the past 10 Ma and the last glacial, challenging the dust–CO₂ linkage. This study addresses three key questions: (1) which iron sources limit EEP productivity and how to distinguish them, (2) whether the EEP has been iron-limited during the Cenozoic, and (3) whether iron inputs, especially via eolian dust, partly regulated atmospheric pCO₂. To investigate these questions, the authors analyze co-registered records of dust input, source-region chemical weathering, and biological pump activity from IODP Site U1333 spanning the late Eocene to early Miocene, including the EOT.

Prior work indicates that iron limits phytoplankton productivity in the EEP (e.g., Kolber et al.) and that experimental iron additions can stimulate large phytoplankton blooms and export. The EEP, as one of three principal iron-limited HNLC regions, has been proposed as a key regulator of atmospheric CO₂ via dust-driven fertilization (the “iron hypothesis”). Conversely, other studies argue that ocean circulation dynamics rather than dust controlled EEP productivity over the past 500 ka, and some reconstructions suggest no significant iron fertilization during the last ice age or over the past 10 Ma, challenging dust’s role in CO₂ sequestration. Conventional productivity proxies (opal, TOC) often suffer from dissolution and remineralization biases in older sediments, limiting their reliability for testing the dust–productivity–CO₂ linkage. Recent advances identify magnetofossils as a robust paleo-productivity indicator sensitive to export productivity and iron supply within sediments near the oxic–anoxic transition, offering a means to reassess iron fertilization impacts over geological timescales. Additionally, marine sediment geochemistry (e.g., rare earth element ratios, Nd isotopes) can distinguish dust provenance (e.g., Asian interior vs. Americas) and infer ITCZ position, while the chemical index of alteration (CIA) provides constraints on source-region weathering intensity that can modulate global carbon cycling.

• Study site and sampling: IODP Site U1333 (present-day EEP HNLC region) Hole B, Section 1H–19X (~170 m thickness). Samples were collected at ~2 m stratigraphic intervals, covering the late Eocene to early Miocene including the EOT.
• Magnetofossil detection and quantification: Measured isothermal remanent magnetization (IRM) acquisition at 80 logarithmically spaced field steps and unmixed components using a 1-D Gaussian mixing model (pyIRM). Acquired first-order reversal curves (FORCs) (up to 201 FORCs; VSM) and processed with VARIFORC to identify magnetically noninteracting single-domain (SD) magnetite typical of magnetotactic bacteria. Transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDXS) on magnetic extracts verified magnetofossil morphologies and distinguished biogenic vs. detrital particles. Anhysteretic remanent magnetization demagnetized at 20 mT (ARM20mT) was used to estimate magnetofossil abundance. Chemical treatment (10 mol/L HCl, 24 h) isolated biogenic and unprotected phases, allowing quantification of SD magnetic inclusion contributions within silicates; magnetofossils accounted for >95% of ARM20mT in representative samples.
• Dust proxies: Determined hard (>300 mT) isothermal remanent magnetization (HIRM300mT) as an indicator of hematite/goethite-rich eolian dust. Conducted sequential chemical extraction to obtain operationally defined eolian dust (ODED), primarily refractory silicates, and evaluated potential volcanic ash contributions using La–Th–Sc geochemistry (found minimal volcanic influence). Measured total Fe, Ti, opal, and TOC to examine relationships between dust/iron supply and export productivity.
• Provenance and ITCZ constraints: Used normalized LaN/YbN ratios to differentiate hemispheric dust sources relative to the mean ITCZ position; all ODED LaN/YbN indicated enriched LREE consistent with Asian interior sources north of the ITCZ. Measured Nd isotopic compositions (εNd) of ODED to assess provenance stability; values fall within the central Asian orogen range, implying a single Asian source over the interval.
• Source-region weathering: Calculated the Chemical Index of Alteration (CIA = [Al2O3/(Al2O3 + CaO* + Na2O + K2O)] × 100; with CaO* corrections for silicate-bound Ca and phosphate correction) on ODED to infer weathering intensity and moisture changes in source regions. Evaluated potential diagenetic, provenance, and sorting effects via CN–A–K trends and comparison with Mongolian dust deposits.
• Age model and fluxes: Ages by linear interpolation on the Westerhold et al. timescale. Fluxes (eolian dust, Fe, TOC, opal, ARM20mT, HIRM300mT) computed using mass accumulation rates (MAR), with carbonate-free MAR to account for carbonate compensation depth variations.
• Instrumentation: Vibrating sample magnetometer (Princeton MicroMag 3900) for magnetic measurements; TEM with EDXS for particle characterization.

• Magnetofossil proxy validation: IRM unmixing and FORC analyses indicate dominance of magnetically noninteracting SD bacterial magnetite in Site U1333 sediments. TEM images show biogenic magnetite alongside irregular detrital particles. Chemical dissolution experiments demonstrate that >95% of ARM20mT is due to magnetofossils, validating ARM20mT as a productivity proxy.
• Productivity patterns: FORC signatures suggest more prominent multi-stranded magnetosome chains during the late Eocene and early Miocene (higher productivity) than during the Oligocene, consistent with magnetofossil abundance tracking export productivity in an iron-limited setting.
• Dust as major Fe source: Strong covariation between total Fe and terrigenous dust indicators (ODED, HIRM300mT; and Ti) indicates that iron supply to the EEP was dominated by eolian dust rather than upwelled sources. Magnetofossil abundance covaries with dust input, supporting dust-driven fertilization.
• Conventional productivity proxies: Opal and TOC show poor correlation with Fe, likely due to dissolution/remineralization and preservation biases, making them less reliable here compared to magnetofossils.
• Dust provenance and ITCZ position: ODED LaN/YbN values fall within the enriched LREE range characteristic of Asian interior dust north of the ITCZ, indicating an Asian dust source reaching Site U1333. This implies that during the Eocene (~45 Ma), the ITCZ lay at least ~4° south of the equator to allow transport to the site at ~3.95°S. Combined with Site U1334 results, the EOT ITCZ northward shift was constrained to at most ~2.5°.
• Single Asian source through time: ODED εNd values lie within the central Asian orogen range, consistent with a single Asian interior source (e.g., Mongolian Gobi) throughout the interval; tectonic/paleogeographic reconstructions further suggest the northern Tibetan Plateau dust source had not formed by the EOT.
• Source-region weathering intensification at EOT: CIA results show a clear shift from weak weathering pre-EOT to moderate weathering post-EOT, indicating a moister central Asian dust source after Antarctic glaciation onset, likely aided by colder conditions, seasonal meltwater, and reduced evaporation.
• Enhanced bioavailable Fe post-EOT: Increased slopes in Fe–Ti and Fe–HIRM300mT relationships after the EOT reflect greater Fe leaching and oxidation (more hematite) from dust due to intensified chemical weathering, implying higher bioavailable iron content in post-EOT dust. Magnetofossil and (to a lesser extent) opal/TOC relations show similar slope increases.
• Dust decline moderated CO₂ drawdown: A pronounced decrease in dust input across the EOT coincides with weakening of the biological pump in the EEP, moderating the global CO₂ decline associated with Antarctic glaciation. Prior to ~34 Ma, superimposed dust fertilization amplified CO₂ removal; after ~34 Ma, reduced dust fertilization lessened CO₂ drawdown efficiency.
• Continental weathering–CO₂ linkage: Synchronous trends between CIA and atmospheric CO₂ from the late Eocene to early Miocene support a key role for intensified continental weathering (linked to central Asian orogenic uplift and humidification) in driving long-term CO₂ decline and fostering icehouse conditions.

The study addresses whether and how eolian dust fertilization influenced EEP productivity and atmospheric CO₂ across a major climate transition. Using magnetofossils as a robust productivity proxy, the authors demonstrate that dust was the dominant iron source to the EEP from the late Eocene to early Miocene, and that enhanced dust supply stimulated the biological pump. Provenance diagnostics (LaN/YbN, εNd) confirm a persistent Asian source and allow inference of ITCZ position and limited migration across the EOT. Source-region CIA reveals a shift to moister, more intensely chemically weathered conditions after the EOT, increasing the potential bioavailable iron fraction in dust. These findings resolve prior ambiguities by linking dust input and composition to productivity and CO₂ trends: before ~34 Ma, dust fertilization amplified atmospheric CO₂ removal; across and after the EOT, a sharp dust decline weakened the EEP biological pump, moderating the global CO₂ decrease even as continental weathering intensified CO₂ drawdown. The interlinked but counteracting processes (long-term tectonic/hydroclimate-driven weathering versus shorter-term dust fertilization changes) underscore the complexity of carbon cycle feedbacks during the greenhouse-to-icehouse transition and highlight the EEP’s sensitivity to dust supply and source-region climate.

Magnetofossils provide a reliable, co-registered proxy for reconstructing dust fertilization and biological pump activity in the EEP over geological timescales. Integrated records from Site U1333 show that: (1) the EEP received iron predominantly from eolian dust; (2) dust fertilization enhanced productivity and CO₂ sequestration prior to ~34 Ma; (3) intensified chemical weathering in central Asian dust source regions augmented atmospheric CO₂ removal across the EOT; and (4) a pronounced dust decline at the EOT weakened the dust-fueled biological pump, moderating the CO₂ decrease during Antarctic glaciation onset. These results reveal complex feedbacks among dust transport, source-region weathering, ocean productivity, and atmospheric CO₂ during a major climate transition.

• Proxy preservation and specificity: Traditional productivity proxies (opal, TOC) exhibit poor correlation with iron due to dissolution/remineralization, limiting their reliability in older records; the study relies instead on magnetofossils. While ARM20mT is validated (>95% magnetofossil contribution), remaining contributions from silicate-hosted magnetic inclusions are possible but were minimized via chemical treatments.
• Dust residue composition: Operationally defined eolian dust (ODED) may include refractory volcanic ash; geochemical screening (La–Th–Sc) suggests minimal volcanic influence in this record, but complete exclusion cannot be guaranteed.
• CIA interpretive constraints: Chemical weathering indices can be affected by provenance changes, diagenesis, and sorting; the authors evaluated and argue against significant impacts using εNd, CN–A–K trends, and comparison to coeval deposits, yet residual uncertainties remain.
• Spatial representativeness: Conclusions are based primarily on a single EEP site (U1333) though contextual comparison with U1334 helps constrain ITCZ shifts; broader spatial coverage could further test regional coherence.