The Mid-Brunhes Event (MBE, or Mid-Brunhes Transition, ~424–478 ka, around the MIS 12–11 boundary) marks a major step-change in Pleistocene climate: post-MBE interglacials became warmer, with higher sea levels, smaller ice volumes, and elevated atmospheric CO₂ (roughly +30–40 ppm baseline shift, from ~240 ppm to ~280 ppm range in interglacials), while glacial CO₂ levels remained comparably low.

The MBE likely resulted from interacting orbital triggers amplified by Southern Ocean processes (sea ice, AABW, AAIW, winds, and freshwater). No single mechanism fully explains the ~35 ppm CO₂ jump; deep and intermediate water changes, plus wind/sea ice feedbacks, worked together.

Researchers from National Taiwan University (led by Dr. Raúl Tapia and Assoc. Prof. Sze Ling Ho) and international partners analyzed sediment cores from the South Pacific. They reconstructed temperature, salinity, and other properties of AAIW (a water mass at ~500–1,500 meters depth) over the past ~600,000 years.

In the Tapia et al. (2026) study and broader paleoceanographic records, AAIW during glacial periods shows characteristic glacial-interglacial variability, but the key long-term shift across the Mid-Brunhes Event (MBE) is primarily in interglacial properties.

Glacial-Interglacial Oscillations

Surface waters (SAMW-influenced): Glacials typically feature cooler sea surface temperatures (SST), as seen in the G. bulloides Mg/Ca proxy at the study site. These follow orbital-scale cycles with larger amplitudes in some intervals.

Subthermocline / AAIW (recorded by G. inflata): Glacials generally align with cooler and often fresher conditions compared to interglacials within each cycle. However, the study highlights that glacial CO₂ levels and associated deep/intermediate water states remained relatively stable across the MBE, unlike the clear step-change in interglacials.

The paper’s reconstructions focus on interglacial AAIW variability (selecting samples from peak interglacials), but downcore data and comparisons (e.g., Fig. 2F in the paper for MIS 2–6) show persistent glacial-interglacial contrasts. Glacial AAIW was generally colder than post-MBE interglacial AAIW.

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Shifts in Antarctic Intermediate Water properties coincide with atmospheric CO₂ rise across the Mid-Brunhes Event

Shifts in Antarctic Intermediate Water properties coincide with atmospheric CO2 rise across the Mid-Brunhes Event is the title of a key 2026 paper published in Science Advances by Raúl Tapia, Sze Ling Ho, and colleagues.

Core Evidence and Methods

The study uses sediment core SO213-60-1 (45°S, 119°W, South Pacific, ~3471 m depth) — a key but data-sparse AAIW formation/source region.

They reconstruct properties over ~600 kyr using:

• Temperature proxies: Mg/Ca ratios (on Globigerina bulloides for surface/SAMW-influenced waters and Globorotalia inflata for subthermocline/AAIW) + independent clumped isotopes (Δ47) on G. inflata for cross-validation. Strong agreement between proxies bolsters confidence.
• Salinity: Ice-volume-corrected δ18O (δ18Osw-IVC) from the same foraminifera.
• Stratification: Vertical thermal gradient (ΔT = SST – SubT).

Key quantitative shifts across the MBE (~424–478 ka):

• Surface temperatures (SST) relatively stable (glacial-interglacial cycles only; minor long-term trend).
• Subthermocline (AAIW) warmed ~5°C post-MBE (pre-MBE avg. ~4°C → post-MBE2 ~9°C).
• Vertical thermal gradient weakened dramatically (~8°C pre → ~2°C post).
• Subthermocline became saltier post-MBE; surface salinity more stable.

Mechanistic Hypothesis

The authors link the shift to reduced Antarctic iceberg meltwater input pre- vs. post-MBE:

• Stronger pre-MBE Antarctic Circumpolar Current (ACC) + more icebergs → greater northward freshwater transport → cooler/fresher surface waters in formation zone → enhanced subduction and stratification.
• Post-MBE: Possible southward shift in Southern Westerly Winds → less ice-shelf melting/iceberg calving + weaker ACC freshwater transport.

This complements (does not replace) deep-water (AABW) changes. Intermediate waters handle a large fraction of ventilated/upwelled water return flow.

AABW remains a cornerstone of Southern Ocean carbon cycle hypotheses for the MBE, emphasizing deep storage/release. The newer AAIW evidence broadens the picture to include intermediate waters as a significant, previously underappreciated player. Together, they highlight multi-layer Southern Ocean dynamics as critical for understanding both past CO₂ shifts and future carbon sink behavior.

The Mid-Brunhes Event is a well-known transition in the Pleistocene where interglacial periods became warmer and had higher CO₂ concentrations, despite broadly similar orbital (Milankovitch) forcing. This research links that atmospheric shift more tightly to Southern Ocean intermediate circulation changes.

The paper is open access and available on Science Advances (DOI: 10.1126/sciadv.ady4567). Press summaries from National Taiwan University and Asia Research News provide accessible overviews.

This work refines our understanding of natural carbon cycle variability and has implications for modeling how the modern Southern Ocean carbon sink might evolve under continued warming and freshening.

Published:  Science Advances

DOI: DOI: 10.1126/sciadv.ady4567

Provided:  National Taiwan University

Authors: Raúl Tapia, Sze Ling, Dirk Nürnberg, A. Nele Meckler, Yoshiyuki Iizuka and Ralf Tiedemann

Abstract

Antarctic Intermediate Water (AAIW) is key to the global carbon cycle, yet its influence on past atmospheric CO₂ changes remains unclear. Using multiproxy reconstructions from the data-poor Pacific sector of the Southern Ocean, we examine interglacial AAIW variability in its source region across the Mid-Brunhes Event (MBE), a major CO₂ transition. While surface temperatures remained stable over 600 thousand years, post-MBE AAIW became warmer and saltier, possibly due to reduced iceberg-derived freshwater input. In contrast, colder, fresher pre-MBE AAIW and enhanced thermal stratification may have promoted greater CO₂ uptake and storage. The post-MBE declining sequestration capacity of AAIW, coinciding with rising atmospheric CO₂, suggests intermediate waters played a critical role in modulating CO₂, challenging the view that changes in bottom-water processes alone controlled this key climatic transition.