the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Sep 2024
New Controls on Sedimentation and Climate in the Central Equatorial Pacific Ocean
Abstract. The equatorial Pacific is a nexus of key oceanic and atmospheric phenomena, and its regional climate has critical implications for hydroclimate, the partitioning of CO2, and temperature on a global scale. The spatial complexity of climate signals across the basin has long posed a challenge for interpreting the interplay of different climate phenomena including changes in the Intertropical Convergence Zone (ITCZ) and El Niño Southern Oscillation. Here, we present new, millennially resolved sediment core chronologies and stable isotope records from three sites in the equatorial Pacific’s Line Islands region, as well as updated chronologies for four previously studied cores. Age constraints are derived from 14C (n=17) and δ18O (n=610), which are used as inputs to a Bayesian software package (BIGMACS) that constructs age models and uncertainty bounds via correlation with the global benthic δ18O stack (Lee et al., 2023). We also make use of the new planktonic δ18O data to draw inferences about surface water salinity and to infer a southward-shifted position for the ITCZ at the Last Glacial Maximum (18–24 ka) and Marine Isotope Stage 6 (138–144 ka). These new chronologies and related datasets improve our understanding of equatorial Pacific climate and show strong promise for further surface and deep ocean paleoclimate reconstructions over the last several glacial cycles.
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RC1: 'Comment on egusphere-2024-2775', Anonymous Referee #1, 28 Jan 2025
This study by Jacobel et al. provides age modeling and stratigraphic information about a series of Central Equatorial Pacific sediment cores. Using new radiocarbon (14C) ages and over 700 new D18O values, dated stratigraphies for three cores, new data for two cores, and context for an additional seven from the Line Islands are presented. The authors use a previously-published modeling tool (BIGMACS) to integrate and align their findings and provide statistical rigor. The study proposes using the coarse fraction (CF%) to better characterize the stratigraphies and to identify possible discontinuities, slumps, or other age inconsistencies in these cores to further improve their usefulness as paleoclimate proxies. Finally, the authors use these findings to identify potential changes in the position of the ITCZ over the last glacial/interglacial cycle based on latitudinal changes in D18O related to precipitation.
The manuscript is well-written and descriptive. It provides excellent background oceanographic and depositional background information for the central equatorial Pacific region around the Line Islands, and it will be a useful addition to the literature. The approach is straightforward and the interpretations follow from the data presented. As one who is familiar with the Line Islands region and one who has used these sediments a great deal, I found myself nodding in agreement with the authors on many points, and appreciate the work here to detail the age models for these cores and demonstrate the utility of such models for paleoclimatic interpretation.
Some detailed items below:
Figure 1 (caption) - the authors reference 72PC, but I didn’t see that labeled on the map. The wording around this is a bit confusing - I believe the authors are saying that is the only other core, in the map but outside of the Line Islands, that has multiple glacial-interglacial cycles? Perhaps that could be clarified.
Age Modeling - Why the additional deltaR uncertainty ? Using the Calib marine reservoir age viewer, there are several “nearby” locations (within 1000km, which, given the spatial extent of the cores, is not overly broad) that provide an average deltaR of ~130 years (but a smaller uncertainty, and are corrected for use with the Marine20 calibration. While this likely doesn’t change the results that much, it would likely provide more precision in the earlier portions of the age models and prevent mis-alignment with cores where such reservoir corrections are used.
(near line 220) - The optimism around core 35BB may be unwarranted. Given the uncertainties between CF%, D18O, and age, and with the observed inversions and sediment mobilization, it would seem that 35BB would require significant effort and meticulous dating to be useful. Opting to not use this for interpretation is prudent.
Line 234 - The line “We refrain from...” Is a bit odd there, since the following section (immediately after that sentence) is the section referenced for more information.
251 - “small radiocarbon reversal” - the magnitude and core depths should be quantified and probably further explained here. The table (Table 1) shows 6 radiocarbon ages in stratigraphic order - is the reversal due to a previous 14C age not in this table? Given this, and the spread of D18O data from 50-0cm (Figure 2), it is important to clarify and quantify this (these?) reversal(s), especially if this core will be used for future LGM-Holocene work. Some additional explanation here is likely in order to detail what section was excised and using what parameters (and do any other sections fit those parameters).
272 -CF% of 16BB is said to generally be decreasing depth. However, the highest CF% is @ 150ka (300cm?), and the distal end of the core (near 250ka, 500cm) is the same (~50%) as the upper 20ka.
340 - the argument for saving wuellerstorfi for B/Ca is made multiple times (see 165, 190).
345 (but also other times) - is the relationship between salinity and D18Osw defined in this paper? Several times the argument is made that enriched D18O is indicative of cooler and/or more saline conditions, but a line or reference that details that relationship would strengthen this claim. Note: this is described later on line 384. Perhaps this relationship should precede the interpretations.
394 (and others, like 426)- here you discuss the “mean state of the El Niño Southern Oscillation” or “ENSO mean state”- but then proceed to discuss the mean conditions of the tropical Pacific (e.g., “weaker zonal SST gradients, weaker winds”). If this is not about ENSO variability (e.g., Thirumalai '24 or Ford '15), where the point is whether ENSO is more or less active (e.g., variability), then would it be more accurate for these conditions to be called the mean state of the tropical Pacific? And while some have hypothesized and/or described a link between tropical Pacific mean state and ENSO variability, being specific here on which is meant would be more precise.
431+ - In my opinion, the interpretation as ITCZ fits the data best, and is consistent with both the data presented and previous work. These two paragraphs describe this relationship and do a convincing job of linking the data with this interpretation. I look forward to sharing this with my students once this is published.
Overall, the authors present a convincing case that ITCZ migration can explain the D18O gradients observed in their cores. The use of a model to integrated 14C and D18O into age models provides uncertainty bounds for the age models. While parts of core 16BB appear to have some stratigraphic ambiguity, these do not appear to have significant implications for the interpretation of ITCZ migration when comparing Holocene and LGM /MIS 6 sections. The minor issues noted are meant to enhance clarity and exposition. Of these, the most important in the scope of this work is detailing the reversals of 16BB, which I believe would strengthen the results.
Citation: https://doi.org/10.5194/egusphere-2024-2775-RC1 -
AC1: 'Reply on RC1', Allison Jacobel, 22 Feb 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2775/egusphere-2024-2775-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Allison Jacobel, 22 Feb 2025
-
RC2: 'Comment on egusphere-2024-2775', Anonymous Referee #2, 14 Feb 2025
The manuscript by Jacobel and co-workers provides a well-presented and well-justified study on sediment cores from the Line Island region. Using new and available stratigraphic data they provide new age model constraints for the studied cores and come up with an interpretation on how the underlying stable oxygen isotope trends along a latitudinal transect are influenced by ITCZ movement.
Overall, this is a well executed study that is easy to read and follow and it provides a nice example that age model construction is the backbone of subsequent paleoclimatic and paleoceanographic reconstructions. Based on the current version, I recommendpublication of the study after some minor changes and corrections.
My main comments and suggestions are (in the way they appear in the text):
1) Figure 1: for someone not familiar with the Line Island region, it would be helpful if the authors could change the inset map to one that shows a broader picture that locates the islands in the central Pacific. Furthermore, core 72PC is mentioned in the caption but it is not shown in the map. Also, the caption says "of the core sites pictured,..." while I do not see any core pictured in the inset.
2) Line 109: also here the cores in the inset are mentioned.
3) Chapter 4.1: Here you mention a reversal in the 14C ages. But in Table 1 there is no reversal visible in the data. Guess these are in the supplemental information, but since this has a prominent meaning in the main text, I would suggest to add the old 14C dates also in Table 1 and lable them accordingly.
4) Lines 252-257: Is there sedimentological evidence that is supporting the deposition of a slump in this part of the core?
5) Chapter 5.1.1: Also here I would recommend to add sedimentological information about the potential slumps and the reworking if there is any observation/data available that could support the atatements in this chapter.
6) Lines 350-370: Here the authors repeat a coupe of times that there is a connection of some sort between the observed amplitude in their data and the sedimentation rates. Furthermore, in lines 366 to 367 they argue exactly the opposite compared to the text before. For me, there is no clear connection between these two things and I would like to ask the authors to revise the text to make the reasoning for their interpretation mor eclear to the reader. Maybe it is something ´very siomple, but I do not get it from the text provided.
7) As a non-native speaker, there seems to be something missing in the sentence of line 374 "...the amplitude of warming the LGM to Holocene..."
Citation: https://doi.org/10.5194/egusphere-2024-2775-RC2 -
AC2: 'Reply on RC2', Allison Jacobel, 22 Feb 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2775/egusphere-2024-2775-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Allison Jacobel, 22 Feb 2025
Status: closed
-
RC1: 'Comment on egusphere-2024-2775', Anonymous Referee #1, 28 Jan 2025
This study by Jacobel et al. provides age modeling and stratigraphic information about a series of Central Equatorial Pacific sediment cores. Using new radiocarbon (14C) ages and over 700 new D18O values, dated stratigraphies for three cores, new data for two cores, and context for an additional seven from the Line Islands are presented. The authors use a previously-published modeling tool (BIGMACS) to integrate and align their findings and provide statistical rigor. The study proposes using the coarse fraction (CF%) to better characterize the stratigraphies and to identify possible discontinuities, slumps, or other age inconsistencies in these cores to further improve their usefulness as paleoclimate proxies. Finally, the authors use these findings to identify potential changes in the position of the ITCZ over the last glacial/interglacial cycle based on latitudinal changes in D18O related to precipitation.
The manuscript is well-written and descriptive. It provides excellent background oceanographic and depositional background information for the central equatorial Pacific region around the Line Islands, and it will be a useful addition to the literature. The approach is straightforward and the interpretations follow from the data presented. As one who is familiar with the Line Islands region and one who has used these sediments a great deal, I found myself nodding in agreement with the authors on many points, and appreciate the work here to detail the age models for these cores and demonstrate the utility of such models for paleoclimatic interpretation.
Some detailed items below:
Figure 1 (caption) - the authors reference 72PC, but I didn’t see that labeled on the map. The wording around this is a bit confusing - I believe the authors are saying that is the only other core, in the map but outside of the Line Islands, that has multiple glacial-interglacial cycles? Perhaps that could be clarified.
Age Modeling - Why the additional deltaR uncertainty ? Using the Calib marine reservoir age viewer, there are several “nearby” locations (within 1000km, which, given the spatial extent of the cores, is not overly broad) that provide an average deltaR of ~130 years (but a smaller uncertainty, and are corrected for use with the Marine20 calibration. While this likely doesn’t change the results that much, it would likely provide more precision in the earlier portions of the age models and prevent mis-alignment with cores where such reservoir corrections are used.
(near line 220) - The optimism around core 35BB may be unwarranted. Given the uncertainties between CF%, D18O, and age, and with the observed inversions and sediment mobilization, it would seem that 35BB would require significant effort and meticulous dating to be useful. Opting to not use this for interpretation is prudent.
Line 234 - The line “We refrain from...” Is a bit odd there, since the following section (immediately after that sentence) is the section referenced for more information.
251 - “small radiocarbon reversal” - the magnitude and core depths should be quantified and probably further explained here. The table (Table 1) shows 6 radiocarbon ages in stratigraphic order - is the reversal due to a previous 14C age not in this table? Given this, and the spread of D18O data from 50-0cm (Figure 2), it is important to clarify and quantify this (these?) reversal(s), especially if this core will be used for future LGM-Holocene work. Some additional explanation here is likely in order to detail what section was excised and using what parameters (and do any other sections fit those parameters).
272 -CF% of 16BB is said to generally be decreasing depth. However, the highest CF% is @ 150ka (300cm?), and the distal end of the core (near 250ka, 500cm) is the same (~50%) as the upper 20ka.
340 - the argument for saving wuellerstorfi for B/Ca is made multiple times (see 165, 190).
345 (but also other times) - is the relationship between salinity and D18Osw defined in this paper? Several times the argument is made that enriched D18O is indicative of cooler and/or more saline conditions, but a line or reference that details that relationship would strengthen this claim. Note: this is described later on line 384. Perhaps this relationship should precede the interpretations.
394 (and others, like 426)- here you discuss the “mean state of the El Niño Southern Oscillation” or “ENSO mean state”- but then proceed to discuss the mean conditions of the tropical Pacific (e.g., “weaker zonal SST gradients, weaker winds”). If this is not about ENSO variability (e.g., Thirumalai '24 or Ford '15), where the point is whether ENSO is more or less active (e.g., variability), then would it be more accurate for these conditions to be called the mean state of the tropical Pacific? And while some have hypothesized and/or described a link between tropical Pacific mean state and ENSO variability, being specific here on which is meant would be more precise.
431+ - In my opinion, the interpretation as ITCZ fits the data best, and is consistent with both the data presented and previous work. These two paragraphs describe this relationship and do a convincing job of linking the data with this interpretation. I look forward to sharing this with my students once this is published.
Overall, the authors present a convincing case that ITCZ migration can explain the D18O gradients observed in their cores. The use of a model to integrated 14C and D18O into age models provides uncertainty bounds for the age models. While parts of core 16BB appear to have some stratigraphic ambiguity, these do not appear to have significant implications for the interpretation of ITCZ migration when comparing Holocene and LGM /MIS 6 sections. The minor issues noted are meant to enhance clarity and exposition. Of these, the most important in the scope of this work is detailing the reversals of 16BB, which I believe would strengthen the results.
Citation: https://doi.org/10.5194/egusphere-2024-2775-RC1 -
AC1: 'Reply on RC1', Allison Jacobel, 22 Feb 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2775/egusphere-2024-2775-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Allison Jacobel, 22 Feb 2025
-
RC2: 'Comment on egusphere-2024-2775', Anonymous Referee #2, 14 Feb 2025
The manuscript by Jacobel and co-workers provides a well-presented and well-justified study on sediment cores from the Line Island region. Using new and available stratigraphic data they provide new age model constraints for the studied cores and come up with an interpretation on how the underlying stable oxygen isotope trends along a latitudinal transect are influenced by ITCZ movement.
Overall, this is a well executed study that is easy to read and follow and it provides a nice example that age model construction is the backbone of subsequent paleoclimatic and paleoceanographic reconstructions. Based on the current version, I recommendpublication of the study after some minor changes and corrections.
My main comments and suggestions are (in the way they appear in the text):
1) Figure 1: for someone not familiar with the Line Island region, it would be helpful if the authors could change the inset map to one that shows a broader picture that locates the islands in the central Pacific. Furthermore, core 72PC is mentioned in the caption but it is not shown in the map. Also, the caption says "of the core sites pictured,..." while I do not see any core pictured in the inset.
2) Line 109: also here the cores in the inset are mentioned.
3) Chapter 4.1: Here you mention a reversal in the 14C ages. But in Table 1 there is no reversal visible in the data. Guess these are in the supplemental information, but since this has a prominent meaning in the main text, I would suggest to add the old 14C dates also in Table 1 and lable them accordingly.
4) Lines 252-257: Is there sedimentological evidence that is supporting the deposition of a slump in this part of the core?
5) Chapter 5.1.1: Also here I would recommend to add sedimentological information about the potential slumps and the reworking if there is any observation/data available that could support the atatements in this chapter.
6) Lines 350-370: Here the authors repeat a coupe of times that there is a connection of some sort between the observed amplitude in their data and the sedimentation rates. Furthermore, in lines 366 to 367 they argue exactly the opposite compared to the text before. For me, there is no clear connection between these two things and I would like to ask the authors to revise the text to make the reasoning for their interpretation mor eclear to the reader. Maybe it is something ´very siomple, but I do not get it from the text provided.
7) As a non-native speaker, there seems to be something missing in the sentence of line 374 "...the amplitude of warming the LGM to Holocene..."
Citation: https://doi.org/10.5194/egusphere-2024-2775-RC2 -
AC2: 'Reply on RC2', Allison Jacobel, 22 Feb 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2775/egusphere-2024-2775-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Allison Jacobel, 22 Feb 2025
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