Day: October 29, 2026

A recent study finds that during the Paleocene- Eocene Thermal Maximum (PETM, ~55.9 million years ago), pelagic fish productivity and carbon export from the surface ocean both increased widely, especially in the Pacific and North Atlantic, contrary to many short- term modern observations and some model projections of productivity declines under warming.

The findings come from a September 2026 open- access paper in Nature Communications (“Coupled increases in pelagic fish productivity and export productivity during the Paleocene- Eocene Thermal Maximum”). The title you referenced matches press coverage of this work (including by science writer David Appell for Phys.org).

Key background and methods

The PETM was a rapid global warming event triggered by a large release of carbon (roughly 3,000- 10,000 Gt C over ~10 kyr), causing a 3–5‰ negative carbon isotope excursion, seawater warming of ~5- 8 °C, ocean acidification, and other stresses. Cumulative carbon release is comparable in scale to high-end future anthropogenic scenarios, though the emission rate was slower, making it a useful long-term analog.

Researchers used:

• Ichthyolith accumulation rates (IAR), fossil fish teeth and scales and dermal denticles in deep- sea sediments, as a proxy for mid- trophic pelagic fish productivity (mainly small pelagic fishes and sharks).
• Marine barite accumulation rates (BAR) to estimate export productivity (new production, Pnew​), because barite forms in association with sinking organic matter.

They examined multiple sediment cores (new data from sites in the North Pacific, Eastern Equatorial Pacific, North Atlantic, South Atlantic, and Southeast Indian Ocean, plus prior data) and compared the proxies spatially and temporally. Earth- system model simulations (cGENIE) under elevated pCO₂ helped interpret mechanisms.

Main results

• IAR rose substantially at Pacific and North Atlantic sites during the main carbon isotope excursion interval, with peaks often 5- 11 times background levels (highest in the Eastern Equatorial Pacific). South Atlantic and Indian Ocean sites showed more modest changes.
• These IAR increases closely tracked rises in BAR- derived export productivity in both timing and spatial pattern. Global pooled data showed a significant positive correlation between the two.
• Model results indicate that under high CO₂, enhanced nutrient supply (e.g., from stronger terrestrial weathering and runoff) can outweigh the productivity-suppressing effects of increased ocean stratification.

The coupled rises suggest that greater carbon export from the surface supplied more food to mid- trophic- level consumers, supporting higher fish productivity. The authors note that higher export in the water column does not automatically mean more organic matter reaches the seafloor.

Implications

Modern satellite and observational data show recent open- ocean productivity declines (especially in low latitudes) linked to stratification and weaker upwelling under warming, and many models project fisheries impacts.

This PETM evidence indicates that the long-term response under sustained greenhouse conditions can differ: productivity and fish production can increase when nutrient inputs rise enough. The authors emphasize that short- term modern trends may not fully capture millennial- to- longer responses once the system equilibrates.

This builds on earlier work showing elevated fish production during early Paleogene warmth (including the Early Eocene Climatic Optimum) and other PETM fish- abundance studies. It adds direct pairing of fish and export-productivity proxies across ocean basins plus modeling support.

In short, the geologic record suggests that ancient ocean warming of this magnitude was associated with surges in both carbon export and pelagic fish productivity, driven in part by nutrient fertilization that overcame stratification effects.

cGENIE is an Earth system model of intermediate complexity commonly used for paleoclimate and carbon- cycle studies.

In this paper, the authors ran a series of transient simulations with it to test possible mechanisms behind the observed rise in export productivity (particulate organic carbon, or POC, flux) during the PETM. They focused on POC export at intermediate depth (~483 m) and compared the results to their barite- derived Pnew​ proxy records.

Experimental design

They performed sensitivity experiments that isolated three main factors:

1. Atmospheric $p$pCO₂ forcing alone (experiment PETM_CP_NER)
   • Atmospheric CO₂ was doubled from a pre- PETM baseline of 1112 ppm to 2224 ppm.
2. CO₂ forcing and temperature- dependent phosphorus (P) weathering (PETM_1×_NER)
   • Added enhanced terrestrial weathering of phosphorus driven by warmer temperatures (consistent with independent lithium-isotope evidence of a ~50–60% global weathering increase).
3. CO₂ forcing and temperature- dependent P weathering and redox- dependent P recycling (PETM_1×_ER)
   • Further added internal phosphorus recycling that strengthens under warmer, more deoxygenated conditions.

They also isolated the individual contributions of each process by differencing the experiments (shown in Fig. 4 of the paper).

Key model results

1. CO₂ doubling / warming and stratification alone produce only a tiny net change

• Global mean POC export shows an initial transient decline (due to stronger stratification reducing nutrient upwelling), followed by gradual recovery.
• At quasi- equilibrium the net increase is only ~0.01 mol m⁻² yr⁻¹, far smaller than the roughly twofold- to- threefold rise inferred from marine barite records.
• The result is insensitive to the exact rate of CO₂ increase.
  → Pure greenhouse forcing and the resulting stratification cannot explain the proxy data.

2. Enhanced weathering -derived phosphorus supply is the dominant driver

• When temperature- dependent P weathering is enabled, ocean- wide average POC export rises substantially: from ~0.7 to ~1.2 mol m⁻² yr⁻¹.
• Stronger weathering enhancements (e.g., 1.9-fold) push the flux even higher (~1.6 mol m⁻² yr⁻¹), matching the magnitude of the global barite-based increase.
• Spatially, the largest modeled increases occur in the Eastern Equatorial Pacific, Atlantic, and Tethys, patterns broadly consistent with the proxy spatial variability.
  → External nutrient input from intensified continental weathering outweighs the negative effects of stratification.

3. Redox-dependent phosphorus recycling adds almost nothing extra

• Turning on stronger internal P recycling on top of the weathering increase produces only a negligible additional ~0.01 mol m⁻² yr⁻¹.
• Reason: in a warmer, more stratified ocean the thermocline deepens and upward nutrient transport weakens, limiting how much recycled phosphorus reaches the surface euphotic zone. This nutrient- transport limitation largely cancels the expected boost from recycling.

4. Regional iron fertilization is a missing amplifier

• The global model does not include regional iron inputs (from North Atlantic Igneous Province volcanism, circum- Caribbean eruptions, or enhanced dust).
• As a result, simulated POC fluxes are generally lower than the highest proxy $P_{\text{new}}$Pnew​ values, especially at the Eastern Equatorial Pacific and North Atlantic sites where iron fertilization is thought to have been important.

Overall interpretation from the model

Under elevated $p$pCO₂, the positive effect of enhanced nutrient supply (mainly via warmer- climate weathering) can outweigh the suppressive effect of ocean stratification on export productivity.

This supports the proxy observation of higher carbon export during the PETM and helps explain why mid- trophic fish productivity also rose. The model also implies that higher export could have acted as negative feedback by increasing organic- carbon burial and helping draw down atmospheric CO₂, although warmer temperatures and acidification may have accelerated organic- matter degradation higher in the water column, so more export does not automatically mean more organic carbon reaches the seafloor.

In short, the cGENIE results show that nutrient fertilization, not just temperature or CO₂, is the key to reconciling increased long- term productivity with a stratified greenhouse ocean.

Coupled increases in pelagic fish productivity and export productivity during the Paleocene-Eocene Thermal Maximum

Abstract (summary)

Ocean productivity is sensitive to climate change. Recent decades show declines, but the long- term response under sustained warming is uncertain. Using paired ichthyolith accumulation rates (IAR; a proxy for pelagic fish productivity) and marine barite accumulation rates (BAR; a proxy for export productivity / new production $P_{\text{new}}$Pnew​), the authors examined the Paleocene-Eocene Thermal Maximum (PETM, ~55.9 Ma). They found a widespread increase in pelagic fish productivity (especially Pacific and North Atlantic) that closely tracks export productivity in both trend and spatial pattern. This suggests the fish increase was fueled by enhanced carbon export from the surface ocean. Earth- system model simulations (cGENIE) show that under elevated pCO₂, enhanced nutrient supply can outweigh the negative effects of ocean stratification on export productivity. The long- term response of marine productivity to sustained greenhouse warming may therefore differ from the short- term declines seen in the modern ocean.

Key context

The PETM involved rapid injection of 3,000- 10,000 Gt of isotopically light carbon, a 3- 5‰ negative carbon isotope excursion (CIE), ocean warming of 5-8 °C, acidification, deoxygenation, and biotic turnovers. Cumulative carbon release is comparable to high- end future anthropogenic scenarios (though the rate was slower), making it a useful long- term analog.

Prior proxies and models for PETM productivity have been inconsistent: BAR and some other records suggested increases (linked to terrestrial nutrient input), while certain microfossil assemblages and models pointed to declines from stratification.

Methods and sites

• IAR: New data from ODP/IODP Sites 1209 (North Pacific), 1221 (Eastern Equatorial Pacific), U1403 (North Atlantic), U1557 (South Atlantic), and U1514 (Southeast Indian Ocean), plus reanalysis of prior data from Site 1220 (Eastern Equatorial Pacific).
• BAR- derived Pnew​: Compilation including new measurements and prior data from matching or nearby sites.
• Comparison with transient and equilibrated cGENIE model experiments testing CO₂ forcing, temperature- dependent phosphorus weathering, and redox- dependent phosphorus recycling.

Main results

• Pronounced IAR increases at Pacific and North Atlantic sites during the main CIE interval (peaks often 5- 11 times as background; highest in Eastern Equatorial Pacific, up to 121 ichthyoliths cm⁻² kyr⁻¹). More modest changes in the South Atlantic and Indian Ocean.
• Many sites show two IAR peaks: one at CIE onset and another ~45- 50 kyr later.
• IAR rises primarily reflect higher ichthyolith flux (not sedimentation- rate changes). Fish community composition remained relatively stable (mostly small teeth <63 μm; no major morphotypes shifts or excursion fauna).
• Pnew​ also rose, with peaks coinciding with IAR peaks in several regions (e.g., Eastern Equatorial Pacific reaching ~187 g C m⁻² yr⁻¹). Global pooled data show a significant positive correlation between Pnew​ and IAR (Pearson $r=0.55$r=0.55, Spearman $\rho =0.61$ρ=0.61).
• Spatial patterns match modern export gradients to a degree, with highest values near upwelling or nutrient- source regions.

Mechanisms (from modeling)

• CO₂- driven warming/stratification alone produces only a small net change in particulate organic carbon (POC) export after an initial decline.
• Enhanced temperature- dependent phosphorus weathering (consistent with lithium- isotope evidence of 50– 60% higher terrestrial weathering) substantially boosts export productivity (roughly matching the observed ~2- 3 times as global rise in some records).
• Redox- dependent phosphorus recycling adds little extra effect once stratification is accounted for.
• Regional amplifiers (e.g., iron from volcanic ash and dust, especially near the North Atlantic Igneous Province) likely contributed in high- productivity areas but were not fully captured in the global model.

Implications

Elevated export productivity increased food availability for mid- trophic pelagic fish, supporting higher fish productivity. The long- term geologic response under sustained high CO₂ can feature productivity increases when nutrient supply rises enough to overcome stratification, contrasting with many short-term modern observations and some century- scale projections.

The paper is open access; the full text, figures, supplementary data, and methods are available at the link. This is the scientific source behind recent coverage (including pieces such as the Phys.org article referenced earlier).

Title: Coupled increases in pelagic fish productivity and export productivity during the Paleocene-Eocene Thermal Maximum

Journal: Nature Communications, volume 17, Article number: 10030 (2026)

DOI: 10.1038/s41467-026-77863-9

Journal: Nature Communications

Authors: Xiuwen Zhou,
Ruiling Zhang,
Man-Yin Tsang,
Mingsong Li &
Weiqi Yao