The modern OMZ in the Arabian Sea is vertically confined between depths of roughly 150 and 1250 m, with the lowest dissolved oxygen levels (<0.5 mL·L⁻¹ or <22 µmol·kg⁻¹) concentrated at depths of 300 - 400 m [7][8]. The intensity and spatial extent of OMZ result due to a mix of high surface productivity, spurred by seasonal upwelling along the Somali and Omani coasts, and restricted ventilation of intermediate waters [9]. Nutrients are introduced into the euphotic zone by the coastal upwelling systems of Somalia and Oman, which are activated by the southwest (summer) monsoon, leading to extensive phytoplankton blooms. The respiration of organic matter sinking into the thermocline and upper intermediate waters then depletes oxygen more rapidly than it can be replenished through physical transport.

Figure 1. Conceptual map showing core sites, OMZ extent, upwelling zones (Somali, Oman), and water masses (AAIW/SAMW, RSW/PGW).

Ventilation of intermediate waters in the Arabian Sea is a complex process influenced by several distinct water masses. From the south, Antarctic Intermediate Water (AAIW) and Sub-Antarctic Mode Water (SAMW) bring oxygen-rich waters to mid-depths [10]. Conversely, in the northern Arabian Sea, Red Sea Water (RSW) and Persian Gulf Water (PGW) introduce warmer, saltier waters with lower oxygen content, which can hinder ventilation [11]. The interplay between these water masses and the oxygen consumption due to remineralization affects the strength and vertical extent of the OMZ (Figure 1).

Figure 2. (A) Conceptual diagram of nitrogen isotope fractionation during water-column denitrification in the Arabian Sea OMZ. The preferential removal of ¹⁴N as N₂ gas enriches residual nitrate in ¹⁵N, which is recorded in sinking particulate organic matter and sediments. (B) Typical δ¹⁵N values for key reservoirs in the marine nitrogen cycle, illustrating the isotopic contrast between weakly and strongly denitrified systems.

Under the suboxic conditions (<5 µmol·kg⁻¹ O₂) prevailing within the Arabian Sea OMZ core, microbial denitrification converts nitrate ( NO 3 − ) to gaseous N₂, representing a major leak in the marine N budget. Estimates suggest that the Arabian Sea accounts for roughly one-third of global water-column denitrification [8][12]. The isotopic fractionation associated with denitrification enriches the residual nitrate δ¹⁵N by 20‰ - 30‰ relative to the substrate [13]. This enriched nitrate is upwelled into the surface ocean and incorporated into phytoplankton biomass, creating a strong isotopic signal that propagates through the food web and is ultimately recorded in sediments [5]. In addition to water-column denitrification, sedimentary denitrification occurs within suboxic porewaters across the Arabian Sea margin. Although sedimentary denitrification also fractionates N isotopes, the effect is typically smaller, and the signal is more susceptible to diagenetic alteration [14]. Distinguishing between water-column and sedimentary denitrification signals in the sedimentary archive remains a key challenge in paleoceanographic interpretation (Figure 2(A), Figure 2(B)).

3.1.1. Bulk Sedimentary δ¹⁵N

Bulk sedimentary δ¹⁵N was initially used as a proxy to reconstruct past denitrification in the Arabian Sea [6][15][16]. The core premise is that elevated δ¹⁵N values signify intensified water-column denitrification and a more pronounced OMZ, whereas lower values suggest reduced denitrification and improved oxygenation. Records of bulk sediment δ¹⁵N from the Oman margin and the eastern Arabian Sea have consistently exhibited a distinct pattern: low values during the LGM, a pronounced deglacial peak between 15 - 12 ka, and higher values throughout the Holocene [15][31][32].

The prevalent interpretation of this pattern suggests a notably reduced or diminished OMZ during the LGM, with a subsequent strengthening of the OMZ throughout the deglaciation and Holocene periods. The decrease in denitrification during the LGM is thought to result from diminished productivity of the southwest monsoon, enhanced ventilation due to the expansion of AAIW, and/or reduced sea levels that limited the entry of low-oxygen waters from marginal seas [23][32].

However, bulk sediment δ¹⁵N is susceptible to several confounding factors: 1) diagenetic alteration during early burial can shift δ¹⁵N values; 2) changes in the relative contribution of terrestrial organic matter, which typically has lower δ¹⁵N (~0 - 3‰), can affect the bulk signal; and 3) the proxy integrates both water-column and sedimentary denitrification signals, complicating attribution [34].

3.1.2. Foraminifera-Bound δ¹⁵N (FB-δ¹⁵N)

To address the diagenetic and mixing challenges linked to bulk sediment δ¹⁵N, researchers have introduced foraminifera-bound nitrogen isotope analysis (FB-δ¹⁵N). This method assesses the δ¹⁵N of organic material enclosed within the calcite shells of planktonic foraminifera, which remain shielded from post-depositional changes and capture the nitrogen isotopic signature of the ocean’s surface or subsurface at the time the shells were formed [17][18].

FB-δ¹⁵N data from the eastern equatorial Pacific (EEP) have questioned the conventional interpretation of bulk δ¹⁵N by revealing similar values during the Last Glacial Maximum (LGM) and the late Holocene. This suggests that the strength of water-column denitrification and the extent of suboxia remained largely consistent between glacial and interglacial periods [17]. Preliminary FB-δ¹⁵N measurements from two locations in the Arabian Sea’s oxygen-deficient zone (ODZ), specifically RC27-24 and RC27-61, dating back to 40,000 years ago, seem to mirror the EEP results. They indicate similar δ¹⁵N values for the LGM and late Holocene, implying comparable denitrification rates and ODZ extents [30]. However, this interpretation requires qualification. FB-δ¹⁵N records the isotopic composition of nitrate assimilated by planktonic foraminifera in the surface or subsurface ocean, which is controlled by several factors in addition to denitrification intensity. These include: 1) nitrate supply and source-waterδ¹⁵N, which depends on the mixing of water masses entering the Arabian Sea (e.g., AAIW, SAMW, RSW); 2) nitrate utilization, since complete consumption of the upwelled nitrate pool dampens the expression of isotopic fractionation associated with denitrification [5]; 3) nitrogen fixation, which introduces isotopically light nitrogen (~0‰) into the surface ocean and can lower the nitrate δ¹⁵N signal independent of denitrification changes [17]; and 4) circulation changes that alter the relative contributions of ¹⁵N-enriched, denitrified waters versus ¹⁵N-depleted waters from the Southern Ocean or marginal seas. Consequently, similar FB-δ¹⁵N values during the LGM and late Holocene may reflect a balance among these processes rather than strictly invariant denitrification rates.

Should these FB-δ¹⁵N findings be validated, they would suggest that the observed glacial-interglacial variations in bulk δ¹⁵N might partly result from diagenesis, shifts in nitrogen fixation, or differences in preservation, rather than directly indicating a significant reduction in water-column denitrification during the LGM. This ongoing discussion highlights one of the most critical unresolved issues in the study of Arabian Sea paleoceanography.

Benthic foraminifera are sensitive indicators of bottom-water oxygenation (BWO) and organic matter flux. The composition, diversity, and species-specific tolerances of their assemblages provide ecological evidence for past oxygenation states that is independent of geochemical proxies [35].

3.2.1. Benthic Foraminiferal Oxygen Index (BFOI) and Enhanced BFOI (EBFOI)

The BFOI, initially created by [20] and later improved (EBFOI); [21] categorizes benthic foraminiferal species based on their oxygen tolerance levels (oxic, suboxic, dysoxic) and computes a numerical oxygen index. In the Arabian Sea’s OMZ, assemblages with high diversity, primarily consisting of oxic indicators like Cibicidoides wuellerstorfi and Oridorsalis umbonatus, suggest well-oxygenated bottom waters. In contrast, assemblages with low diversity, dominated by dysoxic species such as Bolivina spp.,Bulimina spp.,Chilostomella spp., and Fursenkoina spp., indicate oxygen scarcity. Recent research has advanced the BFOI methodology by creating transfer functions that are aligned with current oxygen measurements in the Arabian Sea, facilitating quantitative reconstructions of BWO [21]. These calibrated methods provide dissolved oxygen estimates that can be directly compared with contemporary WOA data and geochemical proxy reconstructions.

3.2.2. Key Indicator Species

Many benthic foraminiferal species serve as particularly sensitive indicators of Arabian Sea oxygenation:

Uvigerina peregrina: An opportunistic species that does well in dysoxic environments with strong organic matter flux. Strong OMZ conditions and bottom-water deoxygenation are indicated by its dominance in Holocene sediments from the Gulf of Oman and the Oman margin [36].Bolivinaspp.: As oxygen levels drop, these taxa become more prevalent because they can withstand low oxygen levels. Increased OMZ conditions are indicated by high Bolivina abundances in mid- to late-Holocene sediments from the northwestern Arabian Sea (ODP Hole 723A) (Academia.edu, 2024).Globobuliminaspp.and Chilostomellaspp.: These infaunal species are indicative of severe oxygen shortage and are typical of severely dysoxic to anoxic settings (Figure 3).

Benthic foraminiferal diversity, measured by the Shannon-Wiener index H(S), is a complementary metric. High diversity indicates well-oxygenated conditions, while low diversity indicates environmental stress due to oxygen depletion (e.g., Gulf of Oman LGM H(S) ~1.94 - 4.27 vs. Holocene decline [29]).

Figure 3. Multi-proxy synthesis of Arabian Sea denitrification and oxygenation over the past 40 kyr. (A) Bulk sedimentary δ¹⁵N showing the characteristic deglacial maximum and elevated Holocene values. (B) Foraminifera-bound δ¹⁵N showing comparatively smaller glacial-interglacial amplitude. (C) Benthic foraminiferal Shannon diversity H(S), declining from LGM high-diversity (oxic) to Holocene low-diversity (dysoxic) assemblages. (D) Reconstructed bottom-water oxygenation (BWO) from multiple proxies, showing the transition from better-ventilated LGM to strongly deoxygenated Holocene conditions. Shaded bars denote climatic periods: LGM (blue), Termination I (orange), Holocene (green).

3.3.1. Iodine-to-Calcium Ratios (I/Ca) in Planktonic Foraminifera

The predominant form of iodine in foraminiferal calcite in oxic conditions is iodate ( IO 3 − ). Iodate is converted to iodide (I⁻), which is incompatible with the calcite lattice, when oxygen levels drop. Thus, I/Ca ratios in planktonic foraminifera are a semi-quantitative indicator of upper-ocean oxygenation; dissolved oxygen contents < 90 µmol/kg are associated with values < 2 µmol/mol [37]. Although its use in deglacial-Holocene sequences is still limited, I/Ca has been applied to Miocene Arabian Sea records [38] and shows promise for Quaternary applications.

3.3.2. Manganese-to-Calcium Ratios (Mn/Ca)

Mn/Ca ratios in bulk sediment or foraminiferal calcite reflect redox shifts because manganese oxides precipitate in oxic conditions and dissolve in suboxic situations. Mn/Ca declines have been used to monitor the Arabian Sea’s gradual deoxygenation during the Miocene [38], and they may give additional limits on glacial-interglacial oxygenation variations.

3.3.3. Uranium and Barium Enrichment Factors (UEF, U/Ba)

Under reducing conditions, soluble U(VI) is reduced to U(IV), which is relatively insoluble and can precipitate or become retained in sediments, resulting in uranium enrichment. In contrast, oxidizing conditions favour the more soluble and mobile U(VI) species. Barium, primarily associated with biogenic barite, is commonly linked to organic-matter export and productivity. Therefore, U enrichment factors (UEF) and U/Ba ratios can provide information on bottom-water redox conditions and oxygenation, particularly where oxygen concentrations and redox conditions fluctuate [23]. In the western Arabian Sea, data from core TN041-8JPC indicate increased bottom-water oxygenation during MIS 6 and decreased oxygenation during MIS 5e based on UEF and U/Ba records.

3.3.4. Benthic Foraminiferal Surface Porosity

The porosity of the surface of benthic foraminifera, especially Cibicidoides and Uvigerina, is linked to the levels of dissolved oxygen in their environment. In conditions where oxygen is scarce, these foraminifera form tests with increased porosity to enhance respiration through the shell wall. This method has been calibrated and utilized to reconstruct bottom water oxygen (BWO) levels in the Arabian Sea, providing estimates of approximately 20 µmol/kg during the Last Glacial Maximum (LGM), which decreased to around 6 µmol/kg during the Holocene in the western Arabian Sea’s oxygen minimum zone (OMZ) [27].

3.3.5. Benthic Carbon Isotope Gradients (Δδ¹³C)

The carbon isotopic difference (Δδ¹³C) between epifaunal (Cibicidoides wuellerstorfi) and deep infaunal (Globobulimina affinis) benthic foraminifera reflects the intensity of porewater organic matter remineralization and, by extension, bottom-water oxygenation. Smaller gradients indicate more oxygenated conditions. Δδ¹³C-based BWO reconstructions from a depth transect (600 - 3650 m) in the Arabian Sea suggest that the glacial OMZ was slightly better oxygenated in the upper 1000 m but that deeper waters (>1500 m) were actually less oxygenated than today [39].

Multi-proxy data suggest that the LGM Arabian Sea was considerably more oxygenated than the Holocene; however, the degree of this difference is still contested.

Isotopic evidence: Sediment δ¹⁵N data from the Oman margin, Murray Ridge, and eastern Arabian Sea generally reveal LGM levels 2‰ - 4‰ lower than Holocene values [15][31][32][40]. Lower δ¹⁵N levels have historically been viewed as indicating decreased denitrification in the water column and a weaker or constricted OMZ. However, new FB-δ¹⁵N data reveal comparable LGM and late Holocene values, raising concerns about the integrity of the bulk sediment signal, although alternative explanations involving changes in nitrate utilization, nitrogen fixation, or source-water mixing cannot be excluded ([30]; see Section 3.1.2).

Ecological evidence: Benthic foraminiferal assemblages from the Gulf of Oman and western Arabian Sea reveal well-oxygenated bottom waters during the LGM. The Gulf of Oman record reveals substantial benthic diversity (H(S) up to 4.27) and reconstructed BWO values that are significantly greater than those from the Holocene. Similarly, ODP Hole 723A (Figure 1) in the northwestern Arabian Sea has an abundance of oxic indicator species during the late Pleistocene, indicating a relatively weak OMZ.

Geochemical evidence: Surface porosity and Δδ¹³ of benthic foraminifera. According to C-based BWO reconstructions, LGM oxygen concentrations were around 10 - 15 µmol/kg greater than present levels in the shallow OMZ (<1000 m), although deeper waters may have been less oxygenated [28]. UEF and U/Ba proxies also show increased BWO throughout glacial eras.

Mechanisms: The better-oxygenated LGM conditions may reflect several competing mechanisms, each supported by proxy evidence: 1) stronger wind-induced mixing due to intensified northeast winter monsoon winds; supported by low bulk δ¹⁵N and reduced TOC [15][32]. 2) expanded northward penetration of oxygen-rich AAIW; consistent with high benthic diversity and expanded AAIW inferred from Δδ¹³C [29]. 3) lower sea levels that disconnected the Persian Gulf and restricted Red Sea Water inflow; suggested by UEF/U/Ba proxies indicating higher BWO when marginal-sea outflow was restricted [27]. 4) weaker southwest summer monsoon upwelling, reducing surface productivity and subsurface oxygen demand, indicated by benthic porosity estimates of ~20 µmol/kg BWO [27][32][41].

The deglaciation is the most dramatic period in Arabian Sea OMZ history, characterized by a strong δ¹⁵N maximum and significant faunal changes.

Isotopic evidence: Sediment δ¹⁵N records from the Arabian Sea show a sharp increase beginning ~18 - 17 ka, reaching maximum values of 8‰ - 10‰ during the Bølling-Allerød (~14.7 - 12.9 ka), before slightly declining through the Younger Dryas and early Holocene [15][16]. This deglacial δ¹⁵N maximum is one of the most durable aspects of the Arabian Sea paleoceanographic records. It is usually understood as reflecting peak denitrification intensity during a period of maximum OMZ expansion.

Ecological evidence: Benthic foraminiferal assemblages exhibit a shift from LGM oxic communities to deglacial dysoxic-dominated assemblages. In the Gulf of Oman, diversity indices deteriorate with deglaciation, and reconstructed BWO levels drop to suboxic levels (<1 mL·L⁻¹) by the early Holocene.

Geochemical evidence: Benthic porosity data show that BWO levels decreased from ~20 µmol/kg during the LGM to ~10 µmol/kg during the deglaciation and stabilized at ~6 µmol/kg during the Holocene [27].

Mechanisms: The deglacial intensification of denitrification and deoxygenation was driven by: 1) strengthening of the southwest summer monsoon in response to rising boreal summer insolation, increasing upwelling and productivity; supported by the bulk δ¹⁵N peak and enhanced TOC burial [15][42]. 2) sea-level rise reconnecting the Persian Gulf (~13 ka) and enhancing Red Sea Water inflow, contributing low-oxygen water masses; indicated by the shift to dysoxic benthic assemblages and declining BWO in porosity records [27]. 3) weakening of AAIW ventilation as Southern Ocean stratification changed; consistent with the timing of Persian Gulf flooding (~13 ka; [43] and increased RSW influence. And 4) rising temperatures reducing oxygen solubility, inferred from rising temperatures and reduced oxygen solubility.

The Holocene is distinguished by chronically high OMZ conditions and enhanced denitrification, but with significant geographical and temporal fluctuations.

Early to Mid-Holocene (11.7-4.2 ka): Bulk δ¹⁵N levels remain elevated with slight changes, indicating continued denitrification. Benthic foraminiferal records from ODP Hole 723A indicate that intense southwest monsoon-driven productivity facilitated OMZ development, but periodic incursions of oxygen-rich intermediate waters from the south (SAMW/AAIW) limited full OMZ intensification during the early Holocene. The Gulf of Oman has consistently low reconstructed O₂ and high δ¹⁵N, with little oscillation, indicating significant OMZ and suboxic bottom water.

Mid-Holocene Transition (~6.5-4.2 ka): The middle Holocene marks a change to more intense OMZ conditions in the northwestern Arabian Sea. This intensification could be attributed to the substitution of oxygen-rich southern-source SAMW/AAIW with oxygen-poor northern Red Sea Water.

Late Holocene (~4.2 ka to present): The Arabian Sea OMZ expanded as the southwest monsoon weakened in the late Holocene. This apparent disconnect between production and oxygenation has been related to the complete halt of oxygenated SAMW/AAIW intrusions, allowing the OMZ to strengthen despite reduced surface forcing. The Gulf of Oman record shows the highest lycopane ratios and Uvigerina peregrina dominance in the late Holocene, indicating maximum deoxygenation [44].

Spatial heterogeneity: The eastern Arabian Sea (Site 722B) reveals orbital-scale denitrification fluctuations closely linked to monsoon severity during the last 800 kyr, with higher δ¹⁵N during interglacials, according to [33]. The southeastern Arabian Sea has centennial-scale denitrification variability overlaid on the glacial-interglacial pattern, indicating high-frequency climate forcing.

High-resolution records show that Arabian Sea oxygenation did not remain constant during the LGM or Holocene, but instead responded to centennial- to millennial-scale temperature oscillations.

MIS 3 (~57-29 ka): This interval has an “unstable” oxygenation mode, alternating between moderately oxygenated circumstances during stadials and deoxygenated/denitrification conditions during interstadials and Dansgaard-Oeschger (D/O) events. In the Gulf of Oman, δ¹⁵N peaks and benthic diversity minima coincide with D/O events, while Heinrich events (H1, H2) and stadials have better oxygenation [44]. Spectral analysis revealed substantial high-frequency periodicities ranging from 570 to 710 years during MIS 3, which may be linked to AMOC variability (Figure 4).

Holocene rapid events: Brief FB-δ¹⁵N maxima at ~8.3, 5.8, and 4.4 ka in regional records may reflect short-lived intensifications of denitrification or changes in thermocline ventilation, although these events require further confirmation in Arabian Sea records [17].

Figure 4. (A) Benthic foraminifera oxygen tolerance categories (oxic, suboxic, dysoxic-anoxic) with representative indicator species for each category, and the Benthic Foraminiferal Oxygen Index (BFOI/EBFOI) scale showing typical LGM and Holocene values. (B) Representative assemblage composition comparing LGM (well-oxygenated, high diversity) and Holocene (deoxygenated, low diversity) states.

The Indian monsoon system is the fundamental driving force behind Arabian Sea surface production and, by extension, subsurface oxygen demand. Southwest (summer) monsoon winds produce coastal upwelling off Somalia and Oman, introducing nutrient-rich waters into the euphotic zone. This interpretation is supported by bulk δ¹⁵N records and benthic faunal assemblages tracking monsoon strength [42][45]. Summer monsoon strength was much weaker during the LGM, strengthened during deglaciation, peaked during the early Holocene “Holocene Climate Optimum,” and weakened again during the late Holocene [42][45]. This monsoon history is closely related to Arabian Sea denitrification intensity: weaker monsoon = lower productivity = less oxygen demand = weaker OMZ (LGM); stronger monsoon = higher productivity = more oxygen demand = stronger OMZ (Holocene). However, the late Holocene paradox shows that diminishing monsoons coincide with intensifying OMZ, which shows that productivity is not the only regulator (Figure 5).

Figure 5. (A) Grouped bar chart comparing representative LGM and Holocene proxy values from the western Arabian Sea core OMZ, illustrating the magnitude and direction of glacial-interglacial changes. (B) Conceptual summary of the LGM state (better oxygenated, weaker OMZ) versus the Holocene state (deoxygenated, stronger OMZ), with key driving mechanisms and the emerging debate between bulk δ¹⁵N and FB-δ¹⁵N interpretations.

The oxygen level of intermediate seas that ventilate the Arabian Sea has a significant impact on OMZ strength, regardless of local production. Glacial expansion of oxygen-rich AAIW is supported by high benthic diversity and oxic indicator species [44], whereas deglacial-Holocene weakening is indicated by declining BWO in porosity [27].

Antarctic Intermediate Water (AAIW) and SAMW: During glacial eras, increased AAIW/SAMW penetration into the northern Indian Ocean resulted in better-ventilated, oxygen-rich intermediate waters [32][41]. During the interglacial period, decreased AAIW production and/or altered routes weakened this ventilation source. The northward spread of AAIW during stadials has been related to higher oxygen levels in the Gulf of Oman during the LGM [44].

Red Sea Water (RSW): Changes in sea level control the outflow of RSW into the Arabian Sea. During the LGM, lower sea levels (~120 m below current) limited RSW outflow across the Bab el Mandeb Strait. During the Holocene, rising sea levels increased RSW influx, bringing warmer, saltier, and lower-oxygen water to the northern Arabian Sea.

Persian Gulf Water (PGW): During the LGM, the Persian Gulf was almost dry and only connected to the Gulf of Oman via the Strait of Hormuz at ~13 ka. Full flooding occurred by ~11.5 ka [43]. Since the mid-Holocene, PGW has helped to ventilate the upper OMZ (150 - 300 m) in the northern Arabian Sea, but its oxygen contribution is minimal (~0.5 mL·L⁻¹ increase; [11][46]).

Post-LGM sea-level rise fundamentally altered Arabian Sea hydrography by: 1) reconnecting marginal seas (Persian Gulf, Red Sea) that contribute distinct water masses; 2) flooding continental shelves, potentially increasing nutrient input from land; and 3) changing coastline configurations that influence upwelling dynamics. The date of Persian Gulf flooding (~13 - 11.5 ka) correlates with the early deglacial δ¹⁵N rise, indicating a potential role for marginal sea connections in OMZ intensification [44].

Warmer temperatures during interglacials decrease oxygen solubility (by ~2 - 3 µmol·kg⁻¹ per ˚C), resulting in thermodynamic feedback that accelerates OMZ expansion. Glacial freezing would have enhanced oxygen solubility, somewhat offsetting any productivity-related oxygen demand [27]. Hypothesized that the minor increase in glacial Arabian Sea OMZ oxygenation was caused by weaker upper-ocean stratification (due to stronger winter monsoon winds) and increased oxygen solubility from lower temperatures, which offset enhanced productivity.

High-resolution records, particularly from the Gulf of Oman, indicate contrasting modes of oxygenation variability over the last ~43 kyr. The relatively stable mode describes intervals in which a dominant long-term oxygenation state persisted despite superimposed centennial- to millennial-scale fluctuations, whereas the unstable mode, most clearly documented during MIS 3, is characterized by pronounced alternations between better-oxygenated and deoxygenated conditions associated with D/O events and AMOC variability [44]. The LGM and Holocene can therefore be considered broadly persistent long-term oxygenation states in the available records, but not strictly constant conditions. Importantly, this framework is currently record-specific and should not be regarded as a basin-wide classification. Its applicability to the Arabian Sea as a whole requires spatially replicated, high-resolution, multi-proxy records that demonstrate consistent temporal patterns across different regions and depths.

The most important methodological challenge in the research is the apparent contradiction between bulk sediment δ¹⁵N (showing significant glacial-interglacial changes) and FB-δ¹⁵N (showing negligible differences). This ongoing discussion highlights one of the most critical unresolved issues in the study of Arabian Sea paleoceanography, and underscores the need to consider non-denitrification controls on FB-δ¹⁵N (Section 3.1.2). Possible theories include:

Diagenetic alteration: During early diagenesis, bulk sediment N may undergo microbial modification, with preferential loss of ¹⁴N-rich compounds. This can artificially elevate or lower glacial bulk δ¹⁵N through multiple mechanisms.N fixation changes: Higher glacial N fixation may have decreased the regional nitrate pool’s δ¹⁵N, decreasing the bulk signal without affecting denitrification.Sedimentary vs. water-column denitrification: Bulk δ¹⁵N may integrate both signals, whereas FB-δ¹⁵N focuses on the water-column signal.Preservationbias: Glacial sediments’ retention of organic materials can impact bulk δ¹⁵N levels.

To resolve this debate, we need more FB-δ¹⁵N records from multiple Arabian Sea sites, paired bulk and FB-δ¹⁵N measurements on the same cores, improved understanding of N cycling and diagenesis in Arabian Sea sediments, and modeling studies that can predict the relative contributions of different N cycle processes.

The Arabian Sea is not a single, homogeneous system. The western Arabian Sea (Oman margin, core OMZ) has the strongest upwelling and denitrification. The Gulf of Oman exhibits unusual behavior, with LGM ventilation patterns that differ from the open Arabian Sea. The eastern Arabian Sea responds to various wind forces and water mass impacts. Future reviews and syntheses must consider this geographical complexity rather than treating the basin as a single entity.

The palaeoceanographic record indicates that Arabian Sea OMZ intensity is affected by: 1) monsoon strength and productivity; 2) intermediate water ventilation from the Southern Ocean; 3) marginal sea connections and sea level; and 4) temperature-driven oxygen solubility. All of these factors are likely to change under future high-emission scenarios (SSP5-8.5): monsoons will intensify with more intense rainfall; ocean warming will diminish oxygen solubility and enhance stratification; and Southern Ocean ventilation will weaken. The paleo-record implies that the Arabian Sea could adopt a stable, substantially deoxygenated phase with extended denitrification, thereby boosting N₂O emissions and affecting global marine productivity. However, the exact trajectory will be determined by the balance of these competing forces.