Evidence for Seawater Retreat With Advent of Meghalayan Era (∼4200 a BP) in a Coastal Harappan Settlement

The transformation of mature (urbanized) phase of the ancient Indus civilisation between ∼4200 and 3800 years Before Present (yr BP) overlaps with the beginning of the Meghalayan Age (∼4200 ± 100 yr BP). Though exact cause(s) for decline of urbanized Indus phase are not yet clear, researchers continue to debate whether monsoonal dryness was the sole cause or several other regional factors manifested in a compounding manner. Here, we show a regional relative sea level fall in the downstream area of Indus habitation (south‐western Gujarat region) which initiated at 4150 ± 230 and continued up to 3625 ± 200 yr BP. We provide a multi‐proxy (chronological, sedimentological, mineralogical, isotopic and elemental abundance) data set from a well‐dated vertical sediment trench from Lothal (ancient dockyard area of Indus era) to support this inference. Chief proxies used for inferring the relative sea level fall were bulk sediment carbon and sulfur contents along with their stable isotopes (δ13C and δ34S) and foraminiferal assemblage. The conspicuous shifts in majority of proxies hint at a lowering of sea stand at the regional level that likely dried this ancient Harappan dockyard (used for sea trade). Findings of our study possess implications for Holocene climate changes and their plausible impact(s) on Harappan trade and culture. Additionally, it invites evidences for large scale geological changes at ∼4200 yr BP distinct to the Meghalayan era.

also witness a regional sea level change? Coastal/estuarine geological records have to be investigated to look for evidence supporting distinctness of this sub-stage. The west coast of India, especially the Gujarat region could serve as a potential location to search for regional sea level changes as it hosts variety of coastal archives, which would aide in reconstructing relative sea level index points (RSL index points ∼ Banerji et al., 2015;Das et al., 2017;Sharma, Chauhan, et al., 2020;Solanki et al., 2021). Along with this the south-western Gujarat is considered as a peripheral region of the Indus valley cultural habitation (Prasad et al., 2014). Early Harappans were known to conduct maritime trade through various ports situated in coastal areas of south-western Gujarat (Gaur & Vora, 1999). These ancient ports are potential locations to investigate the timing of their abandonments and their climatic links. Hence, the major aim of this study was to decipher the position and timing of the relative sea level change in the vicinity of Lothal.
To glean imprints of such geological changes, we provide multi-proxy data set of a well dated sedimentary sequence near the Lothal area, an ancient Harappan dockyard (∼23 km inland from the present-day shoreline). We hitherto provide multiple lines of evidences (in form of geochemical, isotopic, elemental abundance based and foraminifera assemblage as proxies) for a gradual fall in relative sea level from this ancient dockyard area which commenced near the dawn of the Meghalayan age (at ∼4200 yr BP). As the sampled trench lies within the estuarine zone of coastal west coast of India, high frequency changes in sediment lithology were recorded, nonetheless, multiple proxy changes especially (isotopic, elemental abundance and foraminifera assemblage) reveal a likely relative sea level fall which dried the ancient Harappan dockyard and perhaps impeded maritime activities of the mature (urbanized) phase of the Indus era. The mature phase of Indus civilization (4600-3800 yr BP; Sharma, Agnihotri, et al., 2020) is known to have witnessed a significant decline in trade and prosperity after ∼4200 yr BP (Kenoyer, 1998;Meadow, 1993;Possehl, 2002). As long-distance trade with contemporary civilizations was supposed to be a major factor for their advanced technological prowess and prosperity (in addition to their agricultural wealth), probable cause(s) which might have adversely affected their trade have to be investigated in detail. Natural factors that influence maritime trade, such as relative sea level fall, are yet to be explored.

Location and Trench Stratigraphy
The west coast of India, especially the modern-day south-western Gujarat hosted some of the major Harappan coastal trade centers such as Dholavira and Lothal (Dhavalikar et al., 1996;Gaur & Vora, 1999;Khatri & Bhartwaj, 2018;Nigam et al., 2016;Rao, 1979;Vora et al., 2006) (Figures 1a and 1b) along with a few other smaller current landing platforms built for ancient maritime trade (Vora et al., 2006). The Lothal (ancient) dockyard site is located on the banks of the Bhogavo (palaeo river-channel), a Sabarmati tributary, which drains through sandstone, shales and Deccan basalts in its catchment (Merh, 1995). This palaeo-channel was considered active and navigable during the Harappan period, as indicated by the stone anchors and dockyard in Lothal (Khadkikar et al., 2004;Rao, 1979). This riverine route was conjectured to be favored by ancient settlers for transporting artifacts and goods and was eventually linked with the Gulf of Khambhat (Figure 1c). We selected a site (LH site: 22°51'N, 72°23'E; 4 km downstream from the Lothal archeological site) for digging a trench. The selected site has a surface elevation of 7.1 ± 0.1 m above the high water level (ahwl). The site is located at the periphery of the floodplain of the Bhogavo palaeochannel, thus, provides a possibility to assess past hydro-environmental changes ( Figure 1c). The trench site lies in the alluvial plains of the Sabarmati River basin and shows no signatures of intentional human induced changes during the field observations. We made the site selection primarily based on two accounts (a) the downstream position from the Harappan-era dockyard (i.e., at Lothal site) and (b) relatively pristine location (free from human induced geomorphic changes).

Materials and Methods
Due to complexities involved in the sedimentation under shallow marine/ estuarine environment we employed a comprehensive approach comprising variety of physical, geochemical and biotic proxies in this study to decipher various coastal processes (Prasad et al., 2014;Raj et al., 2015;Thakur et al., 2019). As the geographical location of the studied site is marine/fluvial mixed type, organic fraction carbon was not an ideal choice for radiocarbon dating. In view of this, we used foraminiferal 14 C dating by Accelerated Mass Spectrometry (AMS) and also dated few layers containing sand grains using Optically Simulated Luminescence (OSL) dating method. Despite of the relatively larger error for younger time frames, OSL dating is the preferred technique because of its versatility and the availability of the datable material (90-150-micron size quartz in this study), especially, when the datable material for conventional radiocarbon ( 14 C) dating is scarce (Briant & Bateman, 2009;Lee et al., 2011). Conversely, for a particular horizon, where datable material for OSL is scarce and carbon is plausible to extract, the radiocarbon dating is preferred, especially by AMS, as, it provides much better precision compared to the conventional radiometric method (beta decay counting). Radiocarbon dating of bulk sediments often involves issues as the datable carbon may have multiple provenances and, thus, may provide composite age of different carbon components, especially in estuarine environments. Hence, to avoid such complications, the AMS 14 C dating of calcareous planktonic foraminifers is best suited because it provides precise ages (Bard et al., 2004). Another favorable condition for the AMS dating was that the catchment of Bhogavo and Sabarmati rivers does not possess carbonate bedrock, which are one of the culprits for undermining the radiocarbon dating efforts in terrestrial carbonate terrains (Pigati, 2014).
The details of the method followed for the estimating the equivalent dose for optical dating is given by Das et al. (2017) and Prizomwala et al. (2016). On average, 48 aliquots of each sample were analyzed. In general, 20-30 aliquots successfully fulfilled the aforementioned criteria. We used the CAM (central age model) and MAM (minimum age model) (Galbraith et al., 1999;Galbraith & Laslett, 1993) to obtain the most realistic palaeodose estimate. The cosmic ray dose is calculated using the method suggested by Prescott and Hutton (1994) with average water content of 15 ± 5%. The concentrations of Uranium, Thorium, and Potassium were measured using X-ray fluorescence at the XRF laboratory of the Institute of Seismological Research, India. The analysis for AMS 14 C radiocarbon dating was performed at the Poznan Radiocarbon Laboratory, Poznan, Poland, Direct AMS, USA and IUAC, New Delhi, India. Calendric calibration of the AMS 14 C age was accomplished using online version of Calib Rev8.2 programme (Marine 20.14c online software) following data set of Stuiver et al. (2020) utilizing latest marine calibration curve with regional reservoir age correction (ΔR = -106 ± 51 yr). The calibrated ages are expressed as calendar years over a 2σ-error range for OSL and AMS 14 C dating. A statistical approach is used to reconstruct the age-depth model using the Bacon package within the Rstudio software (v.4.1.0) as illustrated by Blaauw and Christen (2011).
Furthermore, to decipher the significant abrupt environmental shifts within the sedimentary record, the multiproxy data set was subjected to the breakpoint analysis using the R statistical software programme. The breakpoints were obtained using the strucchange package within the R-studio (Zeileis et al., 2002(Zeileis et al., , 2003. Major changes in the environmental conditions were marked based on broad agreement of the majority of the breakpoints and a zone was depicted in figures (i.e., Figures 3 and 4: grey zones).

Deconvoluting the Depositional Environment in Sedimentary Record Using Various Proxies
Sediments often carry the signals of processes, characteristics and type of environment experienced by them during their transport and/or deposition (Allen, 2008). In order to assess the sharp environmental change at the dawn of the Meghalayan Age and thereafter, a suite of proxies was employed to distinguish continental versus marine sedimentation. For instance, continental and marine sediments have vast differences not only in their carbon and sulfur contents (Sampei et al., 1997) but also in their stable isotopic fingerprints. The δ 34 S values for seawater sulfate are typically 21.2‰ (Rees et al., 1978). However, the δ 13 C values of seawater carbonates vary between +2‰ and −2‰ (Veizer et al., 1999). In contrast, continental surface soils (especially from western India) have significantly different δ 13 C and δ 34 S values, depending on the dominance of the type of vegetation/ agricultural patterns. δ 13 C and δ 34 S values can range from ∼+10‰ to −27‰ and ∼−3‰ to +5‰, respectively (Deines, 1980). In addition to these aforementioned proxies, we also used temporal distribution of specific elements in the LH sedimentary record. For instance, the ratio of TiO 2 /Al 2 O 3 was used as a proxy for marine influence/processes where TiO 2 is considered to be favored (enriched) in the estuarine zone. Likewise, K 2 O/Al 2 O 3 was used as a proxy for changes in precipitation induced weathering in the region (Bloemsma et al., 2012;Yarincik et al., 2000). Clay mineralogical indices ((Smectite + Kaolinite)/ (Illite + chlorite), Smectite/ (Illite + chlorite)) were used to assess the climatic conditions during the sediment deposition owing to their ability to differentiate the physical versus chemical weathering processes (Alizai et al., 2012;Biscaye, 1965;Khonde et al., 2017;Thamban et al., 2002). Along the lines of aforesaid proxies, elemental ratios such as Ca/Al and Zr/Al could be used for assessing the dominance of marine versus terrestrial processes (Blanchet et al., 2013;Bouimetarhan et al., 2015;Dellwig et al., 2000;Dymond et al., 1992;Fralick & Kronberg, 1997;Revel et al., 2014). Higher Ca/ Al ratios of sediment, would suggest increased marine dominance owing to enriched biogenic Ca contribution by foraminifer shells. This inference was corroborated with actual abundance of Total Foraminiferal Number (TFN) in sedimentary record which is a standard index for assessing the depositional environment (marine/ terrestrial) (Gehrels, 2013). The distribution of Ammonia-Elphidium assemblage, which is a salinity tolerant assemblage and is widely present in shallow marginal marine -brackish environments from different part of the world Debenay & Guillou, 2002;Hardage et al., 2021;Langer & Leppig, 2000;Sen Gupta, 1999) as well as adjacent estuaries of Narmada, Tapi, Mahi (Ghosh, 2012) is another vital proxy to assess change in the estuarine environment from the terrestrial environment.

Sedimentology and Lithostratigraphy of the LH Site
The sedimentological details of the ∼3.8 m deep trench of LH site are shown in Figure 2. The lithostratigraphy of the sequence was divided into six litho-units, based on textural and physical (visual) observations (Folk, 1954) ( Figure 2). A representative number (38) of sediment samples were analyzed for textural variability. Unit 1, which is 26 cm thick, is a dark greenish clayey silt horizon with a higher amount of mica minerals. The overlying unit 2 is a 34 cm thick sandy silt horizon with a gradual decreasing amount of clay and mica minerals. Unit 3 is characterized by a 67 cm thick silt dominated horizon with an intercalated clay layer, which shows parallel laminations with the presence of mica minerals. This is followed by a 15 cm thick (unit 4) silty-sand dominated horizon with clay lenses and a minor amount of mica minerals. The overlying unit 5 is a 79 cm thick clayey silt layer with 0.6 cm thick sand layers. The uppermost part (unit 6) is capped by a 167 cm thick intercalated clayey silt horizon with laminations and mottling.

Chronology
The chronology of the LH trench was constrained using three AMS 14 C ages and two OSL ages (Table 1; Figure 2). We have used all AMS 14 C ages and one OSL bottom most age for the age-depth model (The 2.5 m depth horizon has been dated by both AMS 14 C and OSL, which gave the same ages within 2 sigma; hence we used AMS age for age-depth model owing to its precise nature compared to OSL; Figure 2). The 3.8 m deep sedimentary record provides a depositional history for ∼5100 ± 400-2070 ± 215 yr BP (Figure 2).

Geological/ Environmental Changes During the Deposition History
The sedimentary sequence at LH trench site reveals four distinct depositional environments spanning from ∼5030 yr BP to 2070 yr BP, which are discussed in the following sub-headings.

Phase 1-Pre-Meghalayan Period (∼5030 yr BP to ∼4200 yr BP)
The Pre-Meghalayan period witnessed enhanced concentrations of the sediment carbon (TC) and sulfur (TS) contents along with their stable isotopes. These higher TC and TS concentrations were accompanied with higher concentrations of the TFN (Figure 3). Collectively, these proxies hint that the region had experienced coastal marine environments until 4150 ± 230 yr BP. Imprints of the inferred marine ingression are also evident in the depth profiles of Ca/Al, Zr/Al, and TiO 2 /Al 2 O 3 ratios (see Figure 3).
The hydro-climatic conditions over land for this time window were likely to be arid, as indicated by a conspicuous dip in K 2 O/Al 2 O 3 proxy for precipitation (Figure 4). This inference was also supported by the dominance of Smectite over Illite/Chlorite clay minerals (Figure 4). Based on the breakpoint analysis, a zone is demarcated which showed the transition in the palaeoenvironmental conditions during the pre and post Meghalayan period (Figure 3; gray zone; 4150 ± 230-3625 ± 200 yr BP).

Phase 2 (∼4150-∼3625 yr BP: Late Harappan Period)
The onset of Meghalayan age witnessed abrupt and sharp environmental changes within the region. There are abrupt changes in TC and TS contents along with their isotopes (though gradually) (Figure 3). This sharp change is also supported by an overall decline in TFN along with Ca/Al, Zr/Al and TiO 2 /Al 2 O 3 (Figure 3). Collectively, all aforementioned proxies, hint a conspicuous marine regression at ∼4150 yr BP. Sedimentary sequence deposited after this sharp change were found to be dominated by Ammonia-Elphidium assemblage in the foraminiferal distribution, reflecting the prevalence of the estuarine conditions after ∼4200 yr BP (Figure 3: gray zone). Similarly, the inland conditions at the LH site revealed improved/ wetter climatic conditions, thereby reducing the preceding dryness effect. This is consistent with the distinct positive shifts denoted by the weathering proxies such as TiO 2 /Fe 2 O 3 , K 2 O/Al 2 O 3 ratios along with clay mineralogy.

Phase 3 (∼3675-∼2070 yr BP)
This period (Figure 2: Litho-unit-5 and lower part of −6) indicates overall dry conditions with dip in all the weathering proxies along with the clay mineral assemblage (Figure 4). Observed dryness is also consistent with sharp decline in TFN along with fluctuating TC and TS contents (and their isotopes). Negligible foraminifers were present in this phase, except a few Ammonia-Elphidium Sp. hinting dominantly toward the estuarine-terrestrial conditions with minimal influence of the marine processes. The multi-proxy data for these units collectively indicate the withdrawal of marine ecosystem at the site.

Phase 4 (<2070 yr BP)
This phase is present in the upper part of unit-6 ( Figure 2) and depicts present-day like conditions with lower values of weathering proxies and clay mineral assemblages (Figure 4). The TC, TS contents and their stable isotopes along with Ca/Al, Zr/Al and TiO 2 /Al 2 O 3 also suggests continental conditions at the trench site (Figure 3). This inference is well supported by nearly absent foraminifers in this zone, except for one layer which showed mixed foraminifers (i.e., Ammonia-Elphidium assemblage along with planktonic species like Globigerinoides and Bolivina). A storm surge could bring and transport this mixed assemblage, as storm surges are known to push the offshore derived materials in more landward environments (Collins et al., 1999;Sukumaran et al., 2012). . Down core variations of trace elements with measured Total Carbon (TC%), Total Sulfur (TS%) and their isotopic ratios δ 13 C with δ 34 S value and micropaleontology of the sedimentary succession of the LH trench plotted against burial age determined from Optically Simulated Luminescence (red circle) and Accelerated Mass Spectrometry radiocarbon (purple triangle). The marine influence is usually observed somewhere in and around 3625 ± 200 yr BP, after which the region gradually converted into an estuarine environment up to 2070 ± 215 yr BP, due to the relative fall in the sea level. The demarcated zone is the resultant of the statistically computed breakpoint analysis representing the coherent environmental change observed at the LH site.

Inferred Depositional Environmental Changes at LH Site
Statistical "breakpoint" analysis on the multiproxy data set enabled us to evaluate distinctness of periods before and after the onset of the Meghalayan era (i.e., gray zone of Figure 3). This globally recognized environmental change (Walker et al., 2018) overwhelms the sedimentary depth profiles of the majority of proxy data at the LH site. Depth profiles of TC and TS contents showed changes from <1.5% to >1.5% and <0.1% and >0.1% during the pre-and post-Meghalayan onset (∼4150-3650 yr BP). Similar to shifts in contents, conspicuous changes were also observed for δ 13 C and δ 34 S. We surmise depth profiles of both TC, TS contents along with their isotopic values (δ 13 C, δ 34 S; Figure 3) together with TFN and other geochemical indices (Ca/Al, Zr/Al and TiO 2 /Al 2 O 3 ), suggest presence of the marine environment during the pre-Meghalayan era (between ∼5030 − 4150 yr BP). Both Zr/Al and TiO 2 /Al 2 O 3 ratios showed significant decline above depths of 2.5-3.8 m in the LH trench during Figure 4. Vertical distribution of the major oxides with clay mineral assemblages from the sediments of the LH trench. The sharp change in major oxides and clay mineral assemblages at the gray zone is distinctly marked, which is also in accordance with the statistically computed breakpoint analysis.

Sample ID
Depth ( Depth profiles of TFN also shows a wholesale shift from an average of more than 300 to around 100 before and after a period (∼4150 ± 230 yr BP) that coincides with the arrival of Meghalayan Age (Figure 3).
Difference in depositional environments before and after ∼4150 ± 230 yr BP can also be seen in the clay mineral proxies (Smectite + Kaolinite/ Illite + Chlorite) and precipitation indicator proxies (K 2 O/Al 2 O 3 ) (Figure 4). Proxy data hint a mixed estuarine environment up to ∼2070 ± 215 yr BP. Reading further, the decrease in number of foraminifer tests is also evident at ∼2070 yr BP, with the dominance of Ammonia-Elphidium Sp. assemblage (>50%) which is an euryhaline assemblage. The Ammonia-Elphidium Sp. assemblage widely thrives in shallow marginal marine and brackish environments from the different parts of the world (Debenay & Guillou, 2002;Langer & Leppig, 2000;Sen Gupta, 1999). Estuaries of the Gujarat region such as Narmada, Tapi, Mahi also have revealed dominance of these species (Ghosh, 2012).
Although ∼200 years uncertainty is present in the chronology of the sedimentary data of the LH site, the multiproxy data indicates a major shift circa 4150 yr BP, which approximately coincides with the onset of the Meghalayan Age. Such a comprehensive shift in the depositional environment can be explained by invoking a fall in sea level in the region. The lower part of litho-unit 5 (∼3.8-2.5 m) appears to have deposited under dominant marine environment, whereas the sedimentary section deposited thereafter (between ∼2.5 and 0.6 m) appears to be in a mixed estuarine environment influenced by both fresh water and marine tides, that is, a typical brackish environment.
The overlying litho-unit-6 capturing the period younger than ∼2070 ± 215 yr BP represent prevalence of present-day (dry) conditions. This is evident by lack of foraminifers and other previously discussed proxies ( Figure 3).
Hence, the LH site appears to have been deposited under three deposition environments (a) typical marine (b) mixed-estuarine and the (c) dry (desiccated) environments. The most striking observation is the presence of marine-like conditions at Lothal during Indus civilization which changed right at ∼4150 ± 230 yr BP to mixed-estuarine like conditions which remained up to ∼2070 ± 215 yr BP period. Thereafter, the decentralizing and migrating of Harappan settlers toward the sea may be contemporaneous with monsoonal dryness and regional sea level fall, gradually trending toward the present-day dry conditions ( Figure 5). The inferred relative sea-level fall at the initiation of the Meghalayan Age (∼4150 ± 230 yr BP) might have impeded the functioning of Lothal dockyard, affecting the trade/ business of the Harappan inhabitants which were already suffering from the prevailing monsoonal aridity at the beginning of the Meghalayan Age (∼4200-∼3800 yr BP; Sharma, Agnihotri, et al., 2020).
Several studies have reported that the Lothal dockyard was actively involved in the maritime practices with the Middle East (Gaur & Vora, 1999;Keyoner, 2003;Possehl, 2002;Rao, 1979). This indicates that the Lothal dockyard, despite its ephemeral nature, was navigable for at least a part of the year. Such scenario is only possible when the riverine route experienced high tidal environment, which ensured the presence of water for at least a part of the year in an otherwise present ephemeral river.
The LH site depicts the present-day elevation of 7.1 ± 0.1 m ahwl using Real Time Kinematic Positioning survey while the depth of the estuarine horizon (i.e., limit of tidal ingress) is ∼0.6 m from the top. Based on the available numerical equations (for details see Supporting Information S1), the relative sea level (RSL) and associated error at the LH trench site for marine limiting Hijma et al., 2015;Shennan et al., 2015) stands at 1.2 ± 0.2 m ahwl. However, owing to complexities in tidal range, lack of precise information pertaining to limited tide gauge stations and its implications on sample indicative range, we suggest that the estimate of relative sea level (RSL) should best considered to be approximate in the present case. Thus, the LH site showcases a marine limiting higher sea stand of 1.2 ± 0.2 m ahwl during the pre-Meghalayan era, that is, time window between ∼5030 and 4150 yr BP (Early and Mature Harappan phase) ( Figure 5 and Supporting Information S1). In view of above, we infer that the studied LH site of Lothal, records a plausible relative sea level fall of about 1.2 ± 0.2 m at ∼4200-year BP. This inference is in agreement with the peak of the Middle Holocene high sea stand from the Western India during the 5000-4000 yr BP (Das et al., 2017;Hashimi et al., 1995;Raj et al., 2021;Tyagi et al., 2012). Similar inferences have been drawn from the coasts of El Salvador and Brazil, which witnessed sea stand advancements of 4.7 ± 0.5 m at ∼5600 cal yr BP, followed by a sea retreat at ∼4400 cal yr BP (Martin et al., 2003). Studies from the Rann of Kachchh area of coastal Gujarat, sediment records have revealed a relatively higher sea level from >3000 to 2000 yr BP and a subsequent fall (Makwana et al., 2019). Similarly, Das et al. (2017) reported evidences for a high sea stand during 6000-3000 yr BP period from the Figure 5. A schematic evolution for the LH trench site and surrounding region from 3080 BCE to present. The sea stand reached its peak between 5030 yr BP and 4150 yr BP period. After 4,150 yr BP the relative sea level has gradually started to drop, leading to estuarine conditions at the LH site. Due to the cumulative combination of regressed marine environment and aridity, the shoreline shifted and could no longer be used for ship transportation, ultimately leading to the defunct dockyard of Lothal.
valley fill terrace at the Kharod River, Kachchh. Banerji et al. (2015) studied a mudflat sequence in southern Saurashtra and reported a vertical shoreline change (inclusive of the tectonic component) of ∼2 m between 4710 and 2825 cal yr BP. In a recent study, Solanki et al. (2021) reported a relative fall in sea stand from ∼2.2 m incised terrace which were formed due to Mid-Holocene relative high stand and subsequent fall (during <5500 yr BP). Hence, the timing and extent of high sea stands from these shorelines of western India reveal the spatio-temporal scale of the sea level change during the entire Meghalayan Age and the necessity to document these zones of potential sea level fluctuations.
Taken together, the present study hitherto brings out sea level changes from the vicinity of ancient Indus dockyard (Lothal) during the latter part of the mature phase (urbanized) of the Indus era (∼4200-3800 yr BP) which might have had its impacts on Harappan maritime trade and other related activities. With technical advancements made in the field of stable isotopic measurements, it is possible to simultaneously measure C and S isotopes of few milligrams of geological samples  and in turn detect changes in the regional sea levels. Both east and west coasts of India have plenty of human made structures which today are submerged (Gaur et al., 2021;Sundresh et al., 2014). For instance, east coast of India especially Tamil Nadu coast has rich archeological history and believed that several ancient civilizations were swallowed by the regional sea level changes (Gaur et al., 2021). Our study can potentially rejuvenate interests of archaeologists/ geologists to revisit these sites and record palaeo-sea level fluctuations vis-à-vis climatic perturbations and their eventual impacts on contemporary human habitations.

Conclusions
Based on our multi-proxy (geochemical, isotopic, mineralogical, sedimentological, and biotic) datasets, we present a likely relative sea level fall coinciding with the dawn of the Meghalayan Age (at ∼4150 ± 230 yr BP). Our study clearly provides multi-proxy evidence for a relative sea level fall during the initial phase of the Meghalayan age (∼4150-∼3625 yr BP). This relative sea level fall might have dried ancient Lothal site, the oldest Harappan dockyard. This geological change might have then adversely impacted ancient Harappan trade activities by impeding ship/ boat movements in the vicinity. The Meghalayan era is known to have begun with monsoonal dryness forcing Harappans to migrate toward south-western direction to access water resources. The relative sea level fall between the aforesaid time-window thus might have played a disastrous role for trade activities as well as coastal resources of the ancient civilization. Our study underscores importance of studying ancient maritime hubs to investigate how relative sea level changes (enforced by climatic or tectonic changes) impacted their functionality (or abandonment) using variety of advancements made in dating as well as evaluating hydro-environment using S and its stable isotopic data (δ 34 S) in conjunction with related geochemical proxy data.

Conflict of Interest
The authors declare no conflicts of interest relevant to this study.

Acknowledgments
Financial support from Government of Gujarat is thankfully acknowledged. SAIF, BSIP, Lucknow is thanked for providing the analytical facilities. SPP is thankful to IUAC for extending AMS facility for 14 C funded by Ministry of Earth Science (MoES), Govt. of India with reference numbers MoES/16/07/11(i)-RDEAS and MoES/P.O.(Seismic)8(09)-Geochron/2012. Constructive reviews of two anonymous reviewers and editor helped improve the paper substantially. This is a contribution to IGCP 639 and 725 projects.