Exceptionally high biosphere productivity at the beginning of Marine Isotopic Stage 11

Deglaciations over the last 800,000 years feature rapid increases in atmospheric CO2 of up to ~100 ppm within a few millennia. Proposed mechanisms emphasize Southern Ocean processes (enhanced ventilation, reduced biological export, sea-ice changes, and warmer sea-surface temperatures), yet models cannot fully reproduce the CO2 increases. Meanwhile, terrestrial primary productivity and carbon stocks generally rise during deglaciations, acting as a CO2 sink and implying that additional sources are needed to explain the full deglacial CO2 pattern. Reconstructions of past carbon cycle dynamics draw on δ13C in marine carbonates and atmospheric CO2, but direct information on global productivity (a key carbon flux) is sparse and largely limited to the last deglaciation. Regional oceanic proxies (e.g., TOC/CaCO3 ratios, biomarkers) and terrestrial records (pollen counts, sedimentary TOC) provide valuable but indirect or regionally limited insights. The isotopic composition of atmospheric oxygen (δ18Oatm) has been proposed as a tracer of terrestrial productivity, but it is strongly influenced by the hydrological cycle, complicating its quantitative use. The triple oxygen isotope composition of atmospheric O2 (Δ17O of O2) in ice cores offers a more direct means to estimate global biospheric O2 fluxes, which can be translated to CO2 fluxes via photosynthesis-respiration stoichiometry. Prior Vostok and GISP2 records indicate higher productivity in interglacials than glacials, with interglacial levels similar to today. Termination V, following the Mid-Brunhes event and occurring under low eccentricity, is notable for a long, strong MIS 12 glacial and a long, warm MIS 11 interglacial. Pollen data suggest pronounced terrestrial productivity during MIS 11, and ocean records show unusual carbonate storage, yet the implications for atmospheric CO2 remain unclear. This study aims to generate a high-resolution Δ17O of O2 record from EPICA Dome C across Termination V to reconstruct global biosphere productivity and assess its role in modulating atmospheric CO2 at the onset of MIS 11.

Previous work attributes large deglacial CO2 increases primarily to Southern Ocean processes, including enhanced deep-water ventilation, reduced net organic carbon export, sea-ice retreat, and increased sea-surface temperature. However, model simulations remain controversial and insufficient to explain the full CO2 rise. Terrestrial productivity and carbon stocks increased during deglaciations, implying substantial land CO2 uptake, and necessitating additional sources to reconcile atmospheric patterns. Carbon stock changes have been inferred from compilations of δ13C in marine sediments and atmospheric CO2. Proxies of past global productivity include ocean records of buried organic biomarkers and TOC/CaCO3 ratios (C-rain ratio), but long, continuous records are scarce, particularly in the Southern Ocean. Terrestrial reconstructions (pollen counts, sedimentary TOC) are indirect, regional, and often rely on models. δ18Oatm was proposed as a tracer for terrestrial productivity, but its signal is dominated by hydrological changes and depends on fractionation factors within the water and biosphere cycles. The triple oxygen isotope composition of atmospheric O2 (Δ17O of O2) measured in ice cores has emerged as a direct constraint on global biospheric O2 fluxes, with prior Vostok and GISP2 records covering the last ~400 ka indicating higher interglacial productivity near modern levels. Termination V and MIS 11 have unique features (low eccentricity, strong terrestrial productivity, unusual oceanic carbonate deposition) that call for targeted study using Δ17O of O2 to quantify global productivity changes.

Ice core measurements: Δ17O of O2 was measured in the Antarctic EPICA Dome C (EDC; 75°06′S, 123°21′E, 3233 m a.s.l.) ice core over depths 2735–2797 m, corresponding to ages 405.7–444.1 ka on the AICC2012 chronology, with an average temporal resolution of ~780 years across 50 samples. Raw Δ17O of O2 data were corrected for gravitational fractionation, air bubble trapping fractionation, and gas loss fractionation. Gas loss and bubble trapping corrections were applied here for the first time in this context. To validate consistency with earlier records lacking these corrections, Δ17O of O2 over Termination II was also measured at EDC and compared with the Vostok record; both show a ~51 ppm decrease across Termination II without mean offset due to compensating corrections.

Productivity reconstruction: Global oxygen biosphere productivity was reconstructed following Landais et al. (2007), which estimates Δ17O of O2 produced by terrestrial and oceanic biospheres using published fractionation coefficients for photosynthesis, respiration, and photorespiration. Because the proportions of C3 vs C4 plants and terrestrial vs oceanic productivity vary over glacial-interglacial cycles, Δ17O of O2 produced by the biosphere can change accordingly. Using this framework, pre-industrial (PI), Last Glacial Maximum (LGM), and MIS 11 productivity were estimated, along with uncertainties stemming from fractionation coefficients and the oceanic-to-terrestrial productivity ratio.

Sensitivity tests: To better bound uncertainties, three sensitivity analyses were performed: (1) Photosynthesis fractionation—incorporating the largest observed fractionation during marine photosynthesis (e.g., Emiliania huxleyi) reduced reconstructed MIS 11 productivity by ~3% relative to the no-fractionation assumption. (2) Respiration fractionation—applying a maximum plausible decrease of 0.005 in the slope of ln(δ17O+1) vs ln(δ18O+1) during respiration decreased Δ17Obio by ~65 ppm and yielded a reconstructed MIS 11 productivity ~16% higher than the average case. (3) Water cycle effects—varying meteoric water 17O-excess by −20 ppm during glacial periods and +10 ppm during MIS 11 relative to modern altered reconstructed productivity by ≤6%. Additionally, an alternative reconstruction using the Blunier et al. (2012) model, forced by the new Δ17O of O2 data, was performed for comparison; it produced a smaller glacial–interglacial contrast but still indicated an exceptional productivity increase at the end of Termination V (~10% higher than other interglacials vs ~20% in the Landais et al. approach).

Comparative analyses: The Δ17O of O2 record was compared against atmospheric CO2 across the last five terminations on AICC2012, and against δ18Oatm (EDC) to evaluate contributions from biosphere productivity vs low-latitude hydrology. Regional terrestrial (pollen, TOC, Si/Ti) and oceanic (TOC, alkenone MARs, TOC/CaCO3) records were compiled to contextualize the global signal, with attention to Subantarctic Southern Ocean site PS2489-2/ODP1090 and key Northern Hemisphere pollen sequences.

• Across Termination V, Δ17O of O2 decreased by ~56 ppm, comparable in amplitude to other terminations (IV: 53 ppm; III: 35 ppm; II: 51 ppm; I: 55 ppm), but its decrease was more than twice as long in duration as the CO2 increase (Δ17O decline from ~434.8 to 410.2 ka, 24.6 kyr; CO2 rise from ~434.8 to 424.8 ka, 10 kyr), indicating a pronounced decoupling. • A significant mismatch between CO2 and Δ17O of O2 is evident over Termination V, with the CO2 maximum at ~425 ka occurring >10 kyr before the Δ17O of O2 minimum at ~415 ka; this produces a substantially lower slope in the CO2–Δ17O relationship than for the four younger terminations. • Reconstructed global oxygen biosphere productivity shows glacial periods were ~10–40% lower than interglacials. At the beginning of MIS 11, productivity was exceptionally high, ~10–30% above pre-industrial levels (on average ~17% higher than MIS 1), while the LGM was ~31% lower than MIS 1. • δ18Oatm shows distinctive behavior during Termination V: a larger-than-usual deglacial decrease (~1.8‰ versus <1.5‰ in other terminations) and a high early MIS 11 value (~1.43‰) coincident with peak productivity inferred from Δ17O of O2. This combination indicates an increased terrestrial-to-oceanic productivity ratio. • Southern Ocean Subantarctic records (PS2489-2/ODP1090) suggest a less efficient soft-tissue pump during Termination V (decreases in TOC and alkenone MARs), while the TOC/CaCO3 ratio exhibits an exceptional peak concurrent with a global coccolithophore acme and enhanced reef carbonate accumulation—conditions that would tend to raise atmospheric CO2. • Despite these ocean carbonate signals, atmospheric CO2 at the onset of MIS 11 did not reach the highest interglacial levels and peaked >10 kyr later, coincident with declining global productivity, consistent with a strong biospheric CO2 sink moderating CO2 rise.

The observed decoupling between CO2 and Δ17O of O2 during Termination V implies an anomalously large increase in biosphere productivity relative to other deglaciations. The combination of high Δ17O of O2 and elevated δ18Oatm suggests that this productivity surge was dominated by terrestrial sources rather than oceanic soft-tissue production. Regional terrestrial records (pollen assemblages, prolonged high Si/Ti and TOC in lacustrine sediments) document the expansion and persistence of high-productivity vegetation during Termination V through mid-MIS 11, especially at high northern latitudes, aligning with the global reconstruction. Several mechanisms likely contributed: low orbital eccentricity around 400 ka reducing seasonality and lengthening the growing season; relatively slow sea-level rise preserving vegetated low-latitude platforms; and strong Northern Hemisphere warming and ice-sheet retreat in mid-MIS 11 exposing land and enhancing productivity. In the ocean, Subantarctic indicators point to reduced soft-tissue pump efficiency during Termination V, while exceptional carbonate production by coccolithophores and coral reefs would have promoted CO2 outgassing. The lack of an early MIS 11 CO2 maximum, and the delayed CO2 peak occurring as productivity waned, supports the interpretation that the extraordinary terrestrial biospheric productivity acted as a significant CO2 sink, counterbalancing carbonate-driven CO2 increases. Alternative explanations involving stratospheric ozone changes (e.g., via N2O) do not show similar decoupling during MIS 9 despite comparable N2O levels, reinforcing the productivity-based interpretation.

New high-resolution Δ17O of O2 measurements from the EPICA Dome C ice core across Termination V reveal an exceptional rise in global biosphere productivity at the beginning of MIS 11—approximately 10–30% above pre-industrial, unprecedented over the last four interglacials. The signal, corroborated by δ18Oatm behavior and regional terrestrial records, points to a dominant terrestrial contribution, likely facilitated by low eccentricity (longer summers), slow sea-level rise, and expanded high-latitude land area under warming. This enhanced biospheric sink likely moderated atmospheric CO2 at MIS 11 onset despite strong ocean carbonate production that would otherwise elevate CO2. Future work should target higher-resolution, regionally distributed productivity reconstructions (both terrestrial and oceanic) across Termination V, refine records for recent terminations (e.g., Termination III) and extend analyses to older deglaciations to better constrain the interplay between orbital forcing, biosphere productivity, and atmospheric CO2.

• Reconstruction uncertainties arise from fractionation coefficients in photosynthesis and respiration, the organization of the water cycle, and the time-varying ratio of oceanic to terrestrial productivity. Sensitivity tests indicate impacts of ~−3% (photosynthesis fractionation), ~+16% (respiration fractionation), and ≤6% (water cycle δ17O-excess) on MIS 11 productivity estimates, but these remain sources of uncertainty. • δ18Oatm is influenced by both biosphere productivity and low-latitude hydrology (e.g., monsoon strength), complicating attribution; however, auxiliary records suggest strong monsoon conditions during MIS 11, favoring the productivity interpretation. • Regional terrestrial and oceanic productivity records are sparse, variably dated, and often of limited duration across Termination V, limiting direct global synthesis. • Alternative stratospheric chemistry influences (e.g., ozone changes linked to N2O) could affect Δ17O of O2, though analogous conditions in MIS 9 did not produce similar decoupling, reducing the likelihood of this explanation.