| 15 Jul 2025
Correcting a Systematic Bias in an Ocean Drilling Project Site 882 Alkenone Sea Surface Temperature Record
Abstract. Reconstructions of sea surface temperature (SST) in the geologic record are fundamental to our understanding of Earth’s climate history and the evaluation of Earth’s climate sensitivity to greenhouse gas forcing. SSTs are reconstructed with a variety of methods, including alkenone biomarker lipids produced by certain coccolithophore algae. One such alkenone SST reconstruction from the subpolar northwest Pacific Ocean Drilling Project (ODP) Site 882 has played a large role in shaping the paleoclimate science community’s view of global climate warmth during the Late Pliocene (3.6–2.6 million years ago) and the subsequent cooling that characterized the intensification of Northern Hemisphere Glaciation (Haug, 1995; Haug et al., 2005; Martínez-Garcia et al., 2010). Here, using published data from ODP Site 882 (Studer et al., 2012) and nearby Site 883 (Novak et al., 2024), we demonstrate that the long alkenone SST at ODP Site 882 systematically reports an amplified range of absolute SST values, including maximum SSTs 2–4 °C warmer than the more recently generated data. We suggest that the difference between these datasets is a result of the gas chromatography chemical ionization mass spectrometry (GC-CI-MS) analytical method used by Haug (1995), which is consistent with known challenges with this method (Chaler et al., 2000, 2003). We show that alkenone SST estimates derived from the gas chromatography flame ionization detector (GC-FID) method at Sites 882 and 883 have qualitatively similar trends but are systematically offset in their absolute values from the data first reported from ODP Site 882 (Haug, 1995; Haug et al., 2005; Martínez-Garcia et al., 2010). While this finding does not invalidate the conclusions of the original studies, it does strongly suggest that absolute values derived from published alkenone SST estimates from ODP Site 882 are not suitable for evaluating Earth System Model climate simulations. As an alternative, we present a corrected ODP Site 882 alkenone SST dataset that more closely agrees with the published GC-FID data, albeit with larger uncertainties in the reconstructed SSTs.
Novak and colleagues present research that corrects for biases in previously published alkenone sea surface temperature records at ODP882. These biases stem from differences between analytical techniques for quantifying alkenone concentrations (GC-FID vs GC-CI-MS). The GC-CI-MS method enables analysis of alkenone-poor sediments that were inaccessible with GC-FID. The authors demonstrate that the original GC-CI-MS ODP882 measurements overstated climate variability, though the overall patterns and trends remain unchanged.
While the manuscript is well-written and the research methodology is sound, the manuscript's critical weakness is the absence of a meaningful discussion section. The authors successfully identify and correct the proxy bias, and highlight that the ODP882 record is very important, but fail to explore what this corrected record reveals about our understanding of Pliocene climate and Northern Hemisphere glaciation—which should be a key contribution from this methodological innovation.
MAJOR ISSUE: MISSING DISCUSSION OF IMPLICATIONS
The research convincingly quantifies and corrects the ODP882 bias, but stops short of addressing why this matters for the current understanding of Pliocene temperatures and Northern Hemisphere glaciation. The authors need to address fundamental questions about their findings' significance:
How does this correction alter our understanding of Pliocene temperatures and the timing/intensity of Northern Hemisphere glaciation?
What are the implications for North Pacific temperature evolution during this critical climate transition?
How might this refined record affect estimates of Earth System Sensitivity (ESS), particularly given that ODP882 is frequently cited in multi-proxy compilations?
These questions are only meant to be illustrative and motivate deeper discussion as the current manuscript provides little context for understanding the broader implications of their correction. The paleoclimate community needs to understand not just that the record was biased, but what new insights emerge from the corrected data.
SECTION 2.1
This section could use some more text, particularly since the methods of this manuscript are intertwined with the key message: that the ODP882 record is biased and this is how you quantified and corrected for that bias. In particular, a clearer explanation of how synthetic UK37 were generated is needed for the broad paleoclimate readership of Climate of the Past.
FIGURE 4
A direct comparison between the original and corrected SST records at ODP882 is conspicuously absent. Figure 4 would be the logical place to show this comparison, allowing readers to visualize both the magnitude of the correction and its impact on key climate transitions.
MINOR COMMENT
Without demonstrating the impact of the corrected record, the manuscript somewhat overstates its importance. For instance, the abstract begins by discussing Earth climate sensitivity but many studies of Earth System Sensitivity rely on multiple records, so the authors should either: (a) demonstrate how this correction specifically affects multi-proxy compilations, or (b) focus on what unique insights about regional climate dynamics this corrected record provides.
RECOMMENDATION
This manuscript makes a valuable methodological contribution by identifying and correcting an important bias in a widely used proxy from an important site. However, it currently reads more as a technical note than a full research article. To maximize its impact, the authors must add a robust discussion section that explores what this corrected record teaches us about Pliocene climate that we didn't know before. I enjoyed reading this manuscript and hope the authors find my comments useful.