| 07 Aug 2025
Photic zone niche partitioning, stratification, and carbon cycling in the tropical Indian Ocean during the Piacenzian
Abstract. The mid-Piacenzian Warm Period (mPWP; ~3.264–3.025 Ma) marks the most recent episode of sustained global warmth, characterised by atmospheric carbon dioxide (pCO2) levels similar to those of today. Despite this, our understanding of the vertical structure of the Pliocene ocean and its role in modulating global carbon cycling during this period remains poorly resolved. Here, we combine planktic (coccolith and planktic foraminifera) and benthic (benthic foraminifera) stable carbon (δ13C) and oxygen (δ18O) isotope records from the International Ocean Discovery Program (IODP) Site U1476 in the western tropical Indian Ocean (Mozambique Channel), to reconstruct surface-to-deep ocean conditions during the mPWP. The consistently high vertical δ13C and δ18O gradients indicate long-term thermal stratification and increased carbon export in this moderately elevated pCO2 world. Distinct isotopic signatures observed between the deep-photic zone coccolithophore Florisphaera profunda which dominates the coccolith assemblages, and mid-photic zone planktic foraminifera Globigerinoides ruber suggest ecological partitioning and differing sensitivities to upper ocean dynamics (e.g., stratification, nutrient supply, light intensity). A transient breakdown in stratification and deep ocean carbon storage during Marine Isotope Stage M2 (~3.30–3.28 Ma), a glacial interval preceding the peak warmth of the mPWP, demonstrates the vulnerability of the tropical ocean structure to high-latitude climate forcing. Spectral analysis reveals pronounced obliquity-paced variations in both δ13C and δ18O records, linking high-latitude orbital forcing to carbon cycling in low-latitude regions. These findings offer important new constraints on the ocean–atmosphere carbon feedback during the mPWP and underscore the previously underappreciated role of the tropical Indian Ocean as a dynamic component of global carbon cycling during past warm periods.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Climate of the Past.
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This is an excellent, data-rich contribution that sheds new light on the vertical structure of the tropical Indian Ocean during the Piacenzian. The integration of δ¹³C and δ¹⁸O records from benthic and planktic foraminifera and bulk coccolith fractions, combined with assemblage data, is novel and highly relevant for understanding carbon cycling and stratification in a key low-latitude region.
The manuscript is generally well-written, and the interpretations are thoughtful and supported by the data. The findings on the role of Florisphaera profunda in biasing bulk coccolith isotope signals are particularly important for future proxy studies. The linkage between orbital-scale variability, stratification, and global carbon cycling is compelling.
I recommend minor revisions before acceptance. The main points below aim to improve clarity, strengthen interpretations, and enhance the broader impact of this study.
Major Comments
1. Novelty and Broader Context
The study fills an important spatial gap in Piacenzian reconstructions by providing a tropical Indian Ocean perspective. Emphasizing how these findings complement better-studied Atlantic and Pacific records (e.g., in the Abstract and Conclusions) would highlight the significance of this work for global carbon cycle reconstructions.
2. Age Model and Orbital Phasing
The age model, tuned to LR04, is robust but introduces potential circularity when discussing phase relationships with orbital forcing. Because the phasing between pCO₂, δ¹³C/δ¹⁸O, and insolation is a core conclusion, a brief discussion of age-model uncertainties (e.g., ±kyr at tie-points) and their implications for inferred leads/lags would be valuable (e.g., Section 3.4 and Fig. 4).
3. Coccolith Isotopic Interpretation
The discussion convincingly attributes the lower δ¹³C and heavier δ¹⁸O of the coccolith fraction to the dominance of deep-photic F. profunda. Still, the relative influence of vital effects versus habitat depth and any diagenetic alteration remains qualitative.
4. Productivity and Export Efficiency
The inference that high surface productivity during the mPWP did not lead to efficient export due to stratification is plausible but indirect. Acknowledging the lack of independent export-production proxies (e.g., opal, Ba/Al, %Corg) and noting that this remains a hypothesis would make the discussion more balanced.
5. Conceptual Framework for Orbital Controls
The evidence for obliquity-dominated deep-water variability versus precession-dominated surface variability is compelling. A simple schematic summarizing the proposed mechanisms (linking Southern Ocean ventilation, Indian Ocean stratification, and orbital forcing) would help communicate these insights to a broad readership.
6. pCO₂ and Isotopic Gradients
The discussion of leads/lags between δ¹³C/δ¹⁸O and pCO₂ (Fig. 5) is rich but dense. A brief table summarizing the key intervals (e.g., MIS M2 onset, KM2 event, mPWP peak), the sign of isotopic shifts, and hypothesized drivers (e.g., AMOC weakening, Southern Ocean ventilation) would improve accessibility.
Minor Comments
Recommendation: Minor revisions
This manuscript is a substantial and timely contribution to understanding tropical controls on Pliocene carbon cycling and orbital-scale climate feedbacks. Addressing the points above (particularly clarifying age-model uncertainty, refining coccolith isotope interpretation, and contextualizing productivity–export relationships) will further strengthen an already strong study.