the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Sep 2026
Decadal-scale monsoon seasonality and carbon burial response to the 4.2 ka event in the southwestern Bay of Bengal
Abstract. The monsoon system is the primary driver of agricultural production, ecosystems, and societal development across the Indian subcontinent. However, monsoon’s seasonal response to Holocene climate perturbations remains poorly constrained. The boundary between Northgrippian and Meghalayan ages, defined as the 4.2 ka event, is widely associated with Indian Summer Monsoon (ISM) failure, regional aridification, and the collapse of many civilizations. However, its precise timing, structure, and impact on monsoon seasonality, marine productivity, and carbon burial remains inadequately documented owing to the lack of decadal resolution marine records. Here, for the first time, we present a decadal resolution multiproxy marine record from the southwestern Bay of Bengal, integrating geochemical and planktic foraminiferal proxies to reconstruct the oceanographic response to the 4.2 ka event. We report a two-step decrease in carbon burial, beginning at 4.65 ± 0.17 ka and persisting until 3.97 ± 0.18 ka, spanning ~680 years. The event evolved through two phases, with Phase 1 (4.65–4.45 ka) characterized by a stronger but gradually weakened ISM, a moderate strengthening of the Indian Winter Monsoon (IWM), and enhanced terrestrial organic matter input, followed by Phase 2 (4.45–3.97 ka) marked by a pronounced and sustained ISM weakening and intensified IWM. We propose that the Atlantic Meridional Overturning Circulation triggered the 4.2 ka event, with a complex interaction of Inter Tropical Convergence Zone migration, El Niño-Southern Oscillation variability, and insolation changes operating across different timescales. Our findings establish a mechanistic link between altered monsoon seasonality and reduced carbon burial during the 4.2 ka event, highlighting how a climatically sensitive ocean basin can respond to short-term climatic perturbations.
- Preprint
(6905 KB) - Metadata XML
- BibTeX
- EndNote
Status: open (until 27 Oct 2026)
- RC1: 'Comment on egusphere-2026-4747', Anonymous Referee #1, 21 Sep 2026 reply
-
RC2: 'Comment on egusphere-2026-4747', Anonymous Referee #2, 02 Oct 2026
reply
The Manuscript titled “Decadal-scale monsoon seasonality and carbon burial response to the 4.2 ka event in the southwestern Bay of Bengal," authored by Salman and Saraswat, presents a good dataset and interpretations. The manuscript highlights he Atlantic Meridional Overturning Circulation triggered the 4.2 ka event, with a complex interaction of Inter Tropical Convergence Zone migration, El Niño-Southern Oscillation variability, and insolation changes operating across different timescales. Our findings establish a mechanistic link between altered monsoon seasonality and reduced carbon burial during the 4.2 ka event, highlighting how a climatically sensitive ocean basin can respond to short-term climatic perturbations. The paper is well written, but lots of spelling mistakes, which we normally don’t see nowadays.
It is not the first of its kind study from the Bay of Bengal; earlier on the same timeline, researchers have given their finest work.
Few points are as follows:
Lines 17–19: "for the first time," given the authors' own prior work in this basin and the regional records compiled in Fig. 8, qualify the novelty claim.
Lines 20–25: the abstract should use ± uncertainties consistently with Section 4.1; currently, "4.65 ± 0.17 ka" and "3.97 ± 0.18 ka" appear in the abstract, but "± 165 and ± 175 years" (2σ) in the text (lines 299–301). State which convention is used.
Lines 37, 40: "Gagdil and Gagdil" → Gadgil and Gadgil.
Lines 63–65: "Mayeski et al., 2004" → "Mayewski et al., 2004".
Lines 94–95: "Most of the marine records from the Bay of Bengal don't have the temporal resolution." This is a strong generalization; add the criteria used (sedimentation rate or sampling interval) for the listed records. See the paper by Govil et al., 2022 (JAES).
Lines 173–175: "10 Sv" (line 173) and "~16.301 m³/s" (lines 174–175) appear two sentences apart for comparable freshwater/carbonate fluxes. The latter is overprecise, and the unit format should be m³ s⁻¹. Use consistent units or give both.
Lines 192–195: cite the source of the SST/SSS ranges given here (presumably the World Ocean Atlas climatology of lines 241–243)
Lines 294–297: similarly, "three of the seven radiocarbon dates fall within the event interval itself (4.69 ± 0.17 ka, 4.31 ± 0.18 ka, and 3.96 ± 0.18 ka), directly falling at the onset, middle, and termination" is somewhat a loose statement. 4.69 ka predates the stated onset (4.65 ka) and 3.96 ka is just after the stated termination (3.97 ka). Describe their position relative to the event boundaries precisely, if possible.
Lines 315–317: Corg and Corg/N pre-event means of 2.35% and 9.96, rising to 2.49% and 11.43 in Phase 1, should be given with their uncertainties, as with the other proxies.
Fig. 9 caption Gerber et al.,2024, text says Gerber et al.,2025. In the reference cited, the work is Gerber et al.,2025
Line 736: "Maghalayan" → Meghalayan.
Figures 6 and 7 are confusing because the age is shown in different styles in both figures.
The AMOC and IOD are treated as demonstrated drivers of the event, but neither can be resolved at this site; the record should be reframed around proximal controls and the competing mechanisms already in the literature. The core lies in a semi-enclosed, monsoon-dominated, river-influenced basin in which the upper-water-column structure is governed by monsoon wind strength, the ITF, and freshwater stratification, rather than by the North Atlantic overturning. No AMOC water mass ventilates this site at 509m, so any AMOC signal must reach it through an atmospheric bridge. That bridge is remote, model dependent and slow in the IPSL experiments, an AMOC collapse reduces Indian Monsoon precipitation by only about 0.5 to 2 mm/day over the SW of the subcontinent and the northern BoB, and the authors themselves attributed the connection to an atmospheric bridge with the changes over the Nort Atlantic (Kageyama et al 2009) with a response timescale of the order of a hundred years. The casual direction is also not one-way.
Tropical Indian Ocean warming strengthens AMOC by roughly 5 Sv in the TIO +1 degree C experiment (Hu and Fedorov, 2020) and the AMOC reacts to TIO temperature on monthly to decadal – Fast and Slow multidecadal to centennial timescales with relative TIO index leading an SST-based AMOC proxy by about 26 years since 1870 (Ferster et al., 2023). Meanwhile, the mechanistic literature has moved away from a single North Atlantic trigger. No single forcing run produces double-peak cooling during 4.4-4 cal kyr BP, and there were few eruptions then, which might not have been triggered by external forcing and might have been driven by internal variability (Yan and Liu 2019). There are several studies, such as Bradley and Bakke (2019, noting that solar irradiance varied by no more than ±0.15% and finding no compelling northern North Atlantic anomaly at 4.2 ka. The IOD thing is still described weekly; the IOD-ISMR relationship is asymmetric and phase-dependent, and without a local record sign, the resulting current anomaly cannot be predicted.
Previous work from the western BoB links productivity changes to weaker stratification and enhanced vertical mixing under a weakened SWM, as well as to regional aridification (Pattana et al., 2013; Govil et al., 2022). The discussion should also consider other plausible external and internal forcings, including solar variability and its possible high-latitude feedbacks (Renssen et al., 2006; Swindles et al., 2013), tropical volcanic forcing and its influence on South Asian monsoon convection (Nie et al., 2026), ENSO-related tropical variability (Toth and Aronson, 2019; Dang et al., 2024), and possible non-linear responses to declining insolation (Zanchetta et al., 2016). Govil et al. (2022) further document solar periodicities in the western Bay of Bengal and AMO modulation of the summer ITCZ, while Kageyama et al. (2009), Hu and Fedorov (2020), and Ferster et al. (2023) provide relevant context for a possible AMOC–Indian Ocean teleconnection and its characteristic timescales.
Citation: https://doi.org/10.5194/egusphere-2026-4747-RC2
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 263 | 122 | 52 | 437 | 62 | 52 |
- HTML: 263
- PDF: 122
- XML: 52
- Total: 437
- BibTeX: 62
- EndNote: 52
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
The manuscript Decadal-scale monsoon seasonality and carbon burial response to the 4.2 ka by Salman and Saraswat investigates the monsoon and carbon burial response to the 4.2 ka (Northgrippian – Megalayan boundary in the Holocene) in the Southwest Bay of Bengal. For that, the authors used planktonic foraminifera assemblages and sedimentary geochemical proxies from the core SSD067 PC03.
The manuscript addresses a little studied event in the Holocene time range, making it quite interesting. The text reads well, but the Discussion sub-sections read too long, they should be shortened for a more fluid reading and a better understanding of the importance of discoveries.
The manuscript has two major concerns and some moderate issues that need to be resolved prior to the acceptance, so I recommend major revisions.
The main concern of the manuscript is the age modeling that lacks details in the Material and Methods section and some used criteria to calibrate the ages could not be reliable.
Dated material: which was the planktonic foraminifer mass used to 14C dating? Were sub-thermocline species used to dating? It is important to mention that central (sub-thermocline) water masses could have different 14C reservoirs compared to surface waters.
Age’s calibration: Why did the authors prefer to use the median age in the Calib output instead of to choose the age of the highest mode in the probabilistic curve? Considering the length of the 4.2 ka event (about 600 years) and ages 2σ uncertainties (300 – 400 years), the choice of the best age becomes important, if not crucial to match accurately the event in the age model.
Regarding the age model, how the age model was built? Are the authors used a specific software or linear equations were calculated for each segment of dating points?
Grain size analysis: The methodology used to calculate the coarse fraction is not inserted in the Material and Methods section, but it must be. Besides, it is not clear in the text about which grain size scale the authors defined as coarse fraction. It is shown only in upper panels of the figure 5. Please, clarify in the text. Another issue is about the content of coarse and fine fractions. In lines 411 – 413 the authors stated that the coarse fraction contained only biogenic shells and assumed that lithogenic sediments comprised the silt + clay fractions. What were these statements based on? Please, clarify.
Moderate issues:
Geochemical data: The Corg/N variation seems too low to make inferences about source change in the 4.2 ka event. Such inferences could be improved adding organic bulk δ13C data and building a Corg/N x δ13C diagram (Meyers, 1994).
Z-scores: Although it is a powerful tool for normalizing data from different regions, it follows the assumption that all datasets has normal (Gaussian) distribution. Did the authors carry out a normality test on data? If data does not follow normal distribution, the best choice would be either transforming data or pooling the group of data the authors intend to compare (Andrade, 2021).
Figures
Some figures were built like stratigraphic graphics (Age on y axis) (e.g. figure 4) and others were built as temporal graph (Age on x axis) (e.g. figure 6). Please make all graphics with the same style in order to avoid reader’s misinterpretations. Some figures are called out of order (e.g. Fig. 7 at line 314 that is called before Fig. 5)
Minor issues
Line 86: change “workers” for “works” or “studies”.
Line 89: change “lake” for “lacustrine”.
Line 94: Bay of Bengal don’t have the [enough] temporal resolution.
Lines 105 - 107: please, be more specific. Are the authors writing about absolute (concentration) and relative (percentage) abundances?
Lines 195 and 196: please, italicize “a” after Chlorophyll. Check others entries and correct them, if necessary.
Line 218: up to.
Line 221: Please, full write the acronym “TPN” before its first mention.
Line 222: please, delete “considered”.
Line 255: “Nitrogen content” is not clear. Are the authors measured Total Nitrogen? If yes, use the acronym TN.
Line 294: the ages should be present on figure 3.
Section 3.3: please mention which critical value was used to separate deviated values from mean values and which confidence level.
Line 307: Please, full write the acronym “CF” before its first mention. Add information about this analysis in the methods.
Lines 307 - 308: the Corg/N ratio is not shown on figure 4.
Line 316: please, delete the excess of space in the text
Lines 339 – 341: Neogloboquadrina dutertrei reaches values near to the pre-event in the end of phase 2 according to Figure 6.
Line 351: please, full write the acronym “WMA” prior to the first mention.
Line 362: …the event also varies between different records [and regions]…
Lines 409 – 410: This sentence should be in the Methods section.
Line 423 – 426: this sentence is not clear, please rephrase.
Line 476: Please, delete “The”.
Line 476 – 480: Not clear. The term warm (or cool) summer matches better to the atmosphere. If the authors intends to link to sea surface temperature, they should rephrase those sentences.
Lines 802 and 803: Please, delete [El Niño, La Niña, and associated Southern Oscillation in the equatorial Pacific Ocean] and insert [The ENSO]
Figure 1: Please clarify the season of SW and NE monsoons in the panel (a).
Figure 4: Please, insert the 14C dated ages on the graphic (as symbols) so that readers can verify what is written in the text (e.g. line 294).
Figure 4: in the caption: The Variation[s]
Figure 6: some species of planktic foraminifera have a line and symbol + smoothing plot whereas others species have only symbol + smoothing plot. Please, make all plots with the same layout.
References
Andrade, C. (2021). Z scores, standard scores, and composite test scores explained. Indian journal of psychological medicine, 43(6), 555-557.
Meyers, P. A. (1994). Preservation of elemental and isotopic source identification of sedimentary organic matter. Chemical geology, 114(3-4), 289-302.