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Oceanographic significance of planktonic foraminiferal assemblages in the northwest Indian Ocean-Basin-Ridge system under monsoon influence
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Feng WANG1, Yunhai LI1, 2, **, Xuan DING3, Mingjiang CAI4, Zhikun LAI1, Jian CHEN1, Pengfei SHEN5, Liang WANG1, Xiang YE1
Journal of Oceanology and Limnology | 2026, 44(3) : 990 - 1004
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Journal of Oceanology and Limnology | 2026, 44(3): 990-1004
• Geology •
Oceanographic significance of planktonic foraminiferal assemblages in the northwest Indian Ocean-Basin-Ridge system under monsoon influence
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Feng WANG1, Yunhai LI1, 2, **, Xuan DING3, Mingjiang CAI4, Zhikun LAI1, Jian CHEN1, Pengfei SHEN5, Liang WANG1, Xiang YE1
Affiliations
  • 1Third Institute of Oceanography, Ministry of Natural Resources, Xiamen361005, China
  • 2Laboratory for Marine Geology, Qingdao Marine Science and Technology Center, Qingdao266237, China
  • 3Oceanography Institute, China University of Geosciences (Beijing), Beijing100083, China
  • 4School of Resources and Environmental Science, Quanzhou Normal University, Quanzhou362000, China
  • 5Beijing Institute of Geothermal Research, Beijing100012, China
Published: 2026-05-01 doi: 10.1007/s00343-025-5062-8
Outline
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Planktonic foraminifera in ocean sediments serve as unique paleoenvironmental recorders, and their distribution and diversity are affected by environmental factors. The northwest Indian Ocean, influenced by monsoons and ocean-basin-ridge systems, is a key research area in this regard. However, few studies have focused on the foraminiferal distribution in surface sediments. We analyzed 60 surface sediment samples in April 2023, from which 11 genera and 32 warm-water species were identified. The distribution was primarily controlled by depth-dependent carbonate dissolution, temperature-salinity gradients, and monsoon-driven hydrography. Four distinct assemblages were identified: the Arabian Basin assemblage (dominated by Globorotalia cultrata) demonstrates strong influence from the Arabian Sea high-salinity water and winter monsoon circulation; the Carlsberg Ridge assemblage (dominated by G. cultrata, Globorotalia tumida) exhibits depth-controlled productivity signatures; the Carlsberg Ridge Flanks assemblage (mainly including Globigerinita glutinata, G. cultrata, Globigerinoides ruber) reflects monsoon-induced mixing processes; and the Somali Basin assemblage (including mainly G. cultrata, Pulleniatina obliquiloculata, G. tumida) shows clear imprints of summer monsoon upwelling. Carbonate dissolution in the Somali Basin is strongest, followed by Arabian Basin, the Carlsberg Ridge showed the weakest dissolution. Based on planktonic foraminiferal abundance, fragmentation rate, CaCO3 content, and dissolution susceptible/resistant species, the carbonate lysocline and compensation depth were estimated at ~3 700 and ~4 800 m, respectively.

northwest Indian Ocean  /  surface sediment  /  planktonic foraminifera assemblage
Feng WANG, Yunhai LI, Xuan DING, Mingjiang CAI, Zhikun LAI, Jian CHEN, Pengfei SHEN, Liang WANG, Xiang YE. Oceanographic significance of planktonic foraminiferal assemblages in the northwest Indian Ocean-Basin-Ridge system under monsoon influence[J]. Journal of Oceanology and Limnology, 2026 , 44 (3) : 990 -1004 . DOI: 10.1007/s00343-025-5062-8
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1 　INTRODUCTION
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The northwest Indian Ocean is a distinctive region directly influenced by the monsoonal climatic and oceanographic conditions of the area, including the Arabian Sea, the Somali Sea, and the Carlsberg Ridge. Under the influence of monsoons, different sedimentary systems are formed, and environmental factors such as salinity, temperature, the direction of ocean current migration, and nutrient salts in different geomorphic units constitute sedimentary systems with regional characteristics (Banerjee and Iye, 1991; Schott and McCreary, 2001; Jin, 2004; Liu et al., 2022). Planktonic foraminifera are a significant component of the marine organisms and are an important issue of marine geological study. Their distribution and assemblages are key evidence for the evolution of marine environments. The investigation of distribution patterns of major foraminiferal genera under diverse marine environmental conditions is of great significance in comprehending past and future paleoenvironments and paleoceanography. These assemblages of planktonic foraminifera indicate specific ecological environments, reflecting environmental characteristics like water mass distribution, ocean current conditions, climate change, and biological effects (Illing, 1950; Wang et al., 1988a, b; Banakar et al., 2010; Ravichandran et al., 2021).
Foraminifera have been used in previous studies to make notable contributions to paleoceanography in the northwest Indian Ocean, specifically in relation to long and short-scale changes in the Indian monsoon and thermocline changes (Banakar et al., 2010; Arumugm et al., 2014; Cariño et al., 2023), changes in ancient marine environments (Hermelin, 1992; Rai and Srinivasan, 1994; Ciarapica, 1995; Gupta, 1997; Gupta and Thomas, 1999; Arumugm et al., 2014). Many reports on planktic foraminifera from the Arabian Sea are available (Bé et al., 1971; Brummer and Geert, 2002). These reports mainly focus on the coastal zone near the Arabian Sea, the Somali Basin, or the Carlsberg Ridge, but lack comparative connections among different geomorphic units. We documented the distribution of planktic foraminifera in the northwest Indian Ocean area, including the Carlsberg Ridge area, the Somali Basin, and the Arabian Basin (Barber et al., 2001; Gischler et al., 2003, 2008; Gupt et al., 2006; Boudagher-Fadel, 2008).
Our aim was to reveal the composition of planktonic foraminifera in various marine environments and their distribution in the region in relation to monsoon effects. Ultimately, we hope that our work could lay the foundation for future research into the case in the Indian Ocean. Meanwhile, this study can serve as a corrective approach for interpreting the core records of paleoceanographic changes and unveiling the past monsoon variability from sedimentary archives.
2 　REGIONAL SETTING
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2.1 　Bathymetry
The study area in the northwest Indian Ocean encompasses the Carlsberg Ridge, with the Arabian in the north and the Somali Sea in the south. It spans 50°E–65°E and 0°–20°S, including the hydrothermal field of the Carlsberg Ridge, which is bordered by the Arabian Basin to the north and the Somali Basin to the south. The sampling was carried out at depths from 2 800 to 5 085 m (Ray et al., 2012) (Fig.1).
2.2 　Surface current
Surface currents in the northern Indian Ocean are dominated by the tropical monsoon. During the October to March–April, a strong high-pressure system covered the Asian continent, resulting in the prevalence of the northeast monsoon over the northern Indian Ocean. The sea water in the Bay of Bengal flowed to the southwest and surrounds the island of Sri Lanka in the south, and mixed with the seawater flowing to the southwest of the Arabian Sea, forming the northeast monsoon current (NMC) and west Indian coastal current (WICC). Subsequently, the current flows south along the coast of the Somali Peninsula, forming the Somali Current, and converged with the Northern Equatorial warm current. From the May to September, the southwest monsoon prevailed, resulting in the seawater movement roughly opposite to that in winter, flowing east or northeast. The northern branch of the Southern Equatorial warm current crossed the equator under the influence of the monsoon, entered the northern Indian Ocean, flowed northeast along the coast of Somalia. A Somali current was formed. Upwelling in the northwest Indian Ocean was confined to the coastal zone east of the Somalia and Arabian, forming Arabian Sea high-salinity water (ASHSW) in the Arabian Basin (Banerjee and Iyer, 1991; Kroon et al., 1991; Chen et al., 1994; Prasad et al., 2001; Peng et al., 2015; Rippert et al., 2015).
According to their characteristics, ocean currents can be classified into warm currents and cold currents. In general, the monsoon current in the north Indian Ocean is considered a warm current throughout the year due to its location in the tropics and the high sea surface temperature (SST). This is attributed to the equatorial current, which is also a warm current. However, the ocean currents off the coast of Somalia exhibit an exception to this pattern (Rao and Sivakumar, 1998). During winter, a warm Somali Current is observed, while a Somali Current (cold) is observed during summer. This phenomenon is caused by the prevalence of southwest wind in the coastal waters of the Somali Peninsula during summer, which blows the surface seawater away from the coast, causing the deep seawater to rise due to compensation, forming a strong upwelling, and a significant drop in water temperature. Consequently, the ocean current in this region becomes a cold current (Quadfasel and Schott, 1982; You, 1997; Dahl and Oppo, 2006; Pandey et al., 2015).
2.3 　Distribution of water temperature and salinity
The surface sea temperature in the survey area ranges from 27 to 28 °C. There is a gradual increase in surface sea temperature from the western coastal zone towards the east. At a depth of 50 m, the seawater temperature varies between 24.5 and 26.5 °C, following a similar trend to that observed at the surface. The temperature distribution is centered mainly around the Somali Basin and the Arabian Basin, with temperatures decreasing radially outward. At 120 m, the central temperature of both basins is 21.4 °C, while at 200 m it drops to 15.6 °C. Surrounding areas at these depths exhibit temperatures approximately 1 °C lower (Fig.2).
Regarding salinity, sea surface salinity (SSS) increases from south to north, ranging from 35 to 38.2. Notably, within the ASHSW at a depth of 100 m, the temperature ranges from 24 to 28 °C and the salinity from 35.3 to 36.7 (Fig.3).
3 　MATERIAL AND METHOD
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3.1 　Material
The 60 surface samples were obtained by a box-corer using R/V “Xiangyang Hong 03”, and the sediments are mainly calcareous ooze, and a small number of stations are calcareous clay.
3.2 　Identification of foraminifera
Prior to conducting the foraminifera analysis, the samples were dried at 60 °C and then soaked in clean tap water for 24 h for full dispersion. The soaked samples were repeatedly rinsed using a 63-μm size sieve. After rinsing, the samples were dried at 60 °C. Finally, dried foraminifera of the >150-μm size fraction were identified and counted under an Olympus SZX16 microscope (Kennett, 1983; Darling et al., 2006; Spezzaferri et al., 2015; Brummer and Kučera, 2022).
Fragment %=(number of fragments/8)/((number of fragments/8)+(complete number of planktonic foraminifera))×100 (Le and Shackleton, 1992).
3.3 　Calcium carbonate content
The sample was first freeze-dried for 12 h, and then dried in oven at 60 °C for at least 6 h. 50 mg of the processed sample was weighed and placed in a tinfoil cup for total carbon (TC) content analysis using an elemental analyzer (Vario EL Ⅲ Elemental Analyzer). For organic carbon (OC) analysis, 1 g of the lyophilized and fully ground sediment sample was weighed, mixed with 2 mL of 1-mol/L dilute hydrochloric acid, sonicated for about 3 h, and then heated on a low-temperature electric heating plate for about 12 h. After HCl volatilized, the difference between the total carbon and organic carbon was calculated as inorganic carbon (IC). Standard sample (GSD-9) was used to test the parallel standard samples. The absolute error of three repeated determinations of the element standard samples was less than ±0.3%, and the difference between the analysis results of each element parallel sample was less than 0.6%.
The content of calcium carbonate was calculated based on the content of TC and TOC, using the following formula:
CaCO3=(TC–TOC)×8.33%.
4 　RESULT
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The samples in the study area were dominated by planktonic foraminifera, including 11 genera and 32 species identified, and a small amount of benthic foraminifera. The overall appearance was typical of tropical and subtropical regions. The abundance of planktonic foraminifera ranged from 0 to 26 130/g, with the highest values observed in the Carlsberg Ridge area (8 000–26 130/g). The abundance of planktonic foraminifera in the Arabian Basin was slightly greater than that in the Somali Basin, but both were below 4 000/g. The high-value area of composite divergence of planktonic foraminifera was also observed in the Carlsberg Ridge area (average 1.93), followed by the Somali Basin (average 1.46), and the lowest composite divergence in the Arabian Basin (average 1.16). The planktonic foraminifera fragment rate between 2.72%–86.90%, with an average of 48.69%. The Somali Basin has the highest fragmentation rate (average 63.28%), followed by the Arabian Basin (average 58.13%), and the Carlsberg Ridge has the lowest (average 35.86%). Therefore, we inferred that carbonate dissolution is the strongest in the Somali Basin, followed by the Arabian Basin, and the weakest in the Carlsberg Ridge (Fig.4).
The dominant species having average content of >5% in the planktonic foraminifera assemblage included Globigerinita glutinata, Globorotalia cultrata (Brummer and Kučera, 2022), Globorotalia tumida, Pulleniatina obliquiloculata; those of 3%–5% included Globigerinoides ruber, Trilobatus trilobus (Spezzaferri et al., 2015), Neogloboquadrina dutertrei; and the rest included Globigerina bulloides, Globigerinella calida, Globigerinoides conglobatus, Neogloboquadrina incompta (Darling et al., 2006), Sphaeroidinella dehiscens, etc. These genera and species are common in the tropical-subtropical water masses (Fig.5).
4.1 　Distribution of dominant species
The planktonic foraminifera species and their assemblages reflect environmental characteristics of different sea areas. The distribution traits of main species (Fig.6) are as follows:
Globorotalia cultrata is the most abundant, a large-shelled dissolution-resistant species indicating upwelling (Bé and Hutson, 1977; Berger, 1979; Chen et al., 1991; Chen et al., 1999). It dominates northern Arabian Sea Basin (over 90%), with lower abundances in the south (25% in Carlsberg Ridge ~40% in Somali Basin). Its 79.18% abundance in Arabian Basin correlates with ASHSW from winter northeast monsoon, maintaining high salinity to reduce carbonate solubility.
Globorotalia tumida prefers warmer temperatures (25.3 °C average in Indian Ocean, 2.2 °C higher than G. cultrata) and deeper habitats (Bé and Hutson, 1977; Pflaumann and Jian, 1999; Kawahata et al., 2002; Devendra et al., 2019). It is abundant in Arabian and Somali Basins (max. 53.17%), less so in Carlsberg Ridge and northern Arabian Sea.
Pulleniatina obliquiloculata, a tropical-subtropical thermocline species (Bé and Hutson, 1977; Fairbanks et al., 1982a, 1982b; Thunell et al., 1994; Martinez et al., 1998), peaks in Somali Basin (30%), with distribution dependent on water temperature rather than depth.
Globigerinoides ruber, a widely distributed warm shallow-water soluble species (Bé and Hutson, 1977), appears at 53 stations (0–33.42% abundance), concentrated along northwest-southeast Carlsberg Ridge (often >10%, up to 30%), and is rare in Arabian and Somali Basins at 3 000–4 000-m depth (absent >4 800 m).
Neogloboquadrina dutertrei, a subtropical species favoring high primary productivity in upwelling and continental margins (Bé and Hutson, 1977; Curry et al., 1983; Ravelo et al., 1990; Cannariato and Ravelo, 1997), inhabits subsurface water/thermoclines, with >5% abundance in Carlsberg Ridge, Arabian, and Somali Basins (max. 10%), dropping >4 000-m depth.
Trilobatus trilobus, a warm current indicator in narrow warm-saline environments, shows low abundance (<3% max.) concentrated in southeast Carlsberg Ridge, absent in basin/coastal current areas.
Globigerina bulloides, a cold-water upwelling indicator (Bé and Hutson, 1977; Cannariato and Ravelo, 1997), dominates Carlsberg Ridge, with low abundance in basins/coastal currents (slightly higher in northern Arabian Sea).
Neogloboquadrina incompta, adapted to polar-subpolar cold waters (Curry et al., 1983; Kawahata et al., 2002; Darling et al., 2006), peaks in Somali Basin (10%), rare in the northern Arabian Sea Basin.
Globigerinita glutinata peaks (70%) in the northwestern Somali Basin, coinciding with the summer upwelling zones where monsoon winds enhance nutrient fluxes (Brock et al., 1992). In contrast, G. tumida (5.38% in the Arabian Basin) thrives in warmer waters maintained by ASHSW during winter monsoons, as its thermal preference (25.3 °C) aligns with ASHSW temperatures (Hutson and Prell, 1980; Prell and Curry, 1981).
4.2 　CaCO3 content
The CaCO3 content in the study area exhibits significant spatial variation, ranging from 5.91% to 89.95% with an average of 61.28%. As shown in Figs.7 and 8b, the highest values (exceeding 75%) are concentrated near the Carlsberg Ridge, where shallow water depths (below 3 700 m) and reduced carbonate dissolution create favorable preservation conditions. The Arabian Sea Basin displays moderate CaCO3 contents of approximately 50%, while the Somali Basin shows the lowest values, with concentrations consistently below 25%.
This gradient reflects the combined influence of water depth, monsoon dynamics, and carbonate dissolution. The Somali Basin, with greater water depth, exceeds the carbonate compensation depth (CCD), thereby enhancing dissolution; on the other hand, the strength of the summer monsoon also plays a role, as stronger upwelling introduces nutrient-rich, acidic sub-Antarctic waters, which in turn accelerate carbonate dissolution (Wyrtki, 1973; Kroon and Ganssen, 1989; Conan et al., 2002; Böning and Bard, 2009).
5 　DISCUSSION
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5.1 　Calcite lysocline and calcite compensation depth
It is generally accepted that the interface with CaCO3 content below 10% corresponds to the CCD (Wang et al., 1995). The carbonate lysocline depth (CLD) refers to the depth at which the dissolution rate of calcium carbonate increases abruptly. The vertical distribution of each parameter in the study area is summarized in Figs.7 and 8b. The fragmentation rate remains below 15% above 3 700 m but rises sharply to over 40% at this depth, indicating a significant intensification of dissolution processes. At 3 700 m, the CaCO3 content above this depth is above 75% and drops below 50% below it, further supporting the onset of intensified calcium carbonate dissolution. Within the planktonic foraminiferal abundance, the proportion of dissolution susceptible species exceeds 10% at 3 700 m but is nearly absent in deeper layers, while the proportion of dissolution resistant species increases to over 40%. Below 4 800 m, the fragmentation rate exceeds 80%. When the CaCO3 content falls below 15%, the planktonic foraminifera abundance declines sharply to less than 2 000/g. Based on these observations, it can be inferred that the CLD is near 3 750 m, while the CCD is near 4 800 m. The CLD and CCD in the northwestern Indian Ocean vary regionally and are influenced by factors such as sampling density and external geological conditions. For instance, Shen et al. (2023) reported CLD of 3 900 m in the equatorial western Indian Ocean, while Banakar et al. (2010) observed CLD ranging from 3 500 to 3 800 m in the east Arabian Sea. Our sampling was primarily conducted in the central region of the northwestern Indian Ocean, CLD and CCD align well with previously reported values (Bickert, 2009).
Figure 8a (benthic foraminifera percentage, BFP) and 8b (CaCO3 percentage) collectively reveal the spatial differentiation of carbonate dissolution. The BFP exhibits an exponential increase with water depth (R2=0.68), with 3 500–3 700 m being a critical threshold. In shallow regions, where the mixed layer is shallow and dissolution is weak, planktonic foraminifera are well-preserved (BFP<10%). In contrast, deep areas are leading to significant dissolution of planktonic foraminifera and a BFP rise to >30%. Correspondingly, CaCO3 content decreases exponentially with increasing water depth (R2=0.75). The Carlsberg Ridge, located above the calcite lysocline (CLD), maintains a CaCO3 preservation rate >75%, whereas the Somali Basin, subjected to intense dissolution, shows CaCO3<20%. Therefore, carbonate dissolution is the strongest in the Somali Basin, the weakest in the Carsberg Ridge, and the Arabian Basin in between.
5.2 　Characteristic of planktonic foraminiferal assemblage and sedimentary environment
Correlation analysis of the percentage content of foraminifera can further objectively describe the relationship between the distribution of planktonic foraminifera and the marine environment of the study area (Sun et al., 2003). The R-type cluster analysis was conducted on the main foraminifera species of the study area using software SPSS. The resulting dendrogram could be divided into four groups (Fig.9), and their regional distributions were combined and divided into four assemblages (Fig.10), providing valuable insights into the characteristics of planktonic foraminiferal assemblages in the study area and their environmental indication.
In addition, we conducted a comprehensive analysis by sorting and organizing the content of the primary planktonic foraminifera and relevant background data in four distinct regions, as presented in Tables 1 and 2.
Monsoon circulation affects planktonic foraminiferal distribution by regulating conditions such as temperature, salinity, and nutrients in the ocean, shaping the characteristics of foraminiferal assemblages in different regions:
Arabian Basin (Group 1): high-salinity-low-dissolution environment dominated by northeast monsoon
The Arabian Sea Basin is influenced by the ASHSW driven by the winter northeast monsoon, with salinity of 36.2, which is significantly higher than other sub-regions’ (Table 2). During winter (October–April), the northeast monsoon drives the southward intrusion of ASHSW (salinity 36.2–38.2), triggering coastal upwelling through Ekman pumping (Robinson et al., 2012), which transports deep nitrate (>20 μmol/L) to the surface layer, forming a peak chlorophyll-a zone (1.2 mg/m3). The chlorophyll-a concentration in the core area of ASHSW reached 16.26 μg/dm3 (Fig.11). This nutrient enrichment induced by ASHSW promoted the salt-tolerant species G. cultrata (79.18%) to become the dominant species. Meanwhile, the monsoon-driven horizontal circulation inhibits vertical mixing, while wind stirring maintains surface nutrient concentrations, forming a positive feedback loop of “high salinity high productivity”. Even during the summer southwest monsoon, Ekman pumping in the western Arabian Sea still generates upwelling, bringing nutrients without significantly reducing salinity and continuously maintaining high productivity (high chlorophyll zone shown in Fig.11), which is consistent with the role of monsoons in enhancing primary productivity, indicating that this basin is most strongly affected by winter monsoon forcing.
Carlsberg Ridge (Group 2): warm water-high productivity environment in the monsoon convergence zone
This region is characterized by alternating influences of the Arabian Current and Somali Current under monsoon effects. During the summer southwest monsoon, wind-driven upwelling brings nutrient-rich deep water to the surface, increasing surface silicate concentration, which supports high biological productivity. Its temperature and salinity were greater than those of Group 1, with dominant species including G. cultrata (45.92%), G. tumida (29.08%), P. obliquiloculata (8.24%), and G. glutinata (5.91%). Among them, the abundance of G. tumida is significantly higher than that in Group 1, reflecting its stronger dissolution resistance and preference for warm water; the presence of G. glutinata and P. obliquiloculata may indicate monsoon-driven upwelling. In addition, the high abundance of G. bulloides clearly indicates ongoing upwelling in this region, and also enhances the diversity of the assemblage. Overall, the foraminiferal assemblages and oceanographic characteristics show that this region is an upwelling zone with high biological productivity.
Carlsberg Ridge Flanks (Group 3): preservation status in shallow waters and upwelling-driven productivity
The foraminiferal assemblage in the flanks of the Carlsberg Ridge dominant species included G. glutinata (17.41%), G. cultrata (17.23%), and G. ruber (15.07%), with T. trilobus (8.05%). The abundance of G. glutinata is significantly higher than those in the areas around, reflecting the intrusion of Subantarctic water brought by monsoon-driven upwelling, which is confirmed by elevated chlorophyll-a level (Fig.11). Combined with the relatively shallow water depth (compared to Groups 1 and 2) G. ruber, T. trilobus, and G. glutinata that contribute to the preservation, it could be determined that this region is a warm and high-productivity upwelling zone with excellent carbonate preservation.
Somali Basin (Group 4): warm water-high productivity environment under intense carbonate dissolution
During summer (May–September), offshore winds driven by the southwest monsoon trigger coastal upwelling, bringing nutrient-rich Subantarctic Mode Water and Antarctic Intermediate Water (water temperature<25 °C, salinity 35.5) to the surface (Fairbanks et al., 1982; Schott and McCreary, 2001; Schott et al., 2009), which increased surface silicate concentration (>15 μmol/L) and formed an eastward chlorophyll-a gradient (from 1.0 mg/m3 in the west to 0.8 mg/m3 in the east). The chlorophyll-a concentration near the upwelling core area (58.57°E, 12.03°N) was 4.67 μg/dm3, and decreased to 3.09 μg/dm3 in the areas more distant from coast (61.81°E, 12.88°N). Consequently, the content of P. obliquiloculata (accounting for 18.90%) increased, and G. glutinata reached the highest abundance (70%) in the northwest. However, due to the great water depth of the basin and the CO2 enrichment brought by upwelling, carbonate dissolution was intensified (Böning and Bard, 2009), resulting in a fragmentation rate of 87% and carbonate content <20%, showing the characteristics of “high productivity-high dissolution-weak preservation”.
The dominant species in the study region were the dissolution-resistant species G. cultrata (41.05%) and G. tumida (18.58%), and the abundances are similar to those of Group 2, reflecting the overall warm water characteristics, and these species could survive under extreme dissolution conditions. The large presence of P. obliquiloculata (18.90%) and N. dutertrei (5.92%) indicates that the upwelling signal could still be retained even under strong dissolution pressure.
6 　CONCLUSION
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In this study, through the identification of foraminifera in 60 surface samples of the northwest Indian Ocean and the discussion of the water depth and ocean current in the study area, the following main conclusions were drawn:
(1) The surface sediments contained 11 genera and 32 warm-water planktonic foraminiferal species, representing a typical assemblage characteristic. High abundance values were observed in relatively shallow areas located on the sea ridge.
(2) Dominant species, with an average relative abundance exceeding 5% in the planktonic foraminiferal assemblage, include G. glutinata, G. cultrata, G. tumida, and P. obliquiloculata.
(3) Four distinct planktonic foraminiferal assemblages have been identified: (a) the Arabian Basin assemblage, which reflects the influence of ASHSW and winter monsoon circulation; (b) the Carlsberg Ridge assemblage, which exhibits productivity patterns controlled by water depth; (c) the Carlsberg Ridge Flanks assemblage, which indicates monsoon-induced mixing processes; and (d) the Somali Basin assemblage, which displays clear signatures of summer monsoon-driven upwelling.
(4) Based on planktonic foraminiferal abundance, fragmentation rates, CaCO3 content, and dissolution-susceptible/dissolution-resistant species, the CLD and CCD were estimated at approximately 3 700 m and 4 800 m, respectively.
7 　DATA AVAILABILITY STATEMENT
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Data inquiries can be directed to the corresponding author.
8 　ACKNOWLEDGMENT
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We thank all the crew members for their help in collecting samples and data onboard “Xiang Yanghong 3” during surveys. We also thank Ms. Yijun LAN (Third Institute of Oceanography, Ministry of Natural Resources) and Ms. Wenjuan SU (Third Institute of Oceanography, Ministry of Natural Resources) for their help in geochemical experiments.
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doi: 10.1007/s00343-025-5062-8
  • Receive Date：2025-03-03
  • Online Date：2026-07-29
  • Published：2026-05-01
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  • Received：2025-03-03
Affiliations
    1Third Institute of Oceanography, Ministry of Natural Resources, Xiamen361005, China
    2Laboratory for Marine Geology, Qingdao Marine Science and Technology Center, Qingdao266237, China
    3Oceanography Institute, China University of Geosciences (Beijing), Beijing100083, China
    4School of Resources and Environmental Science, Quanzhou Normal University, Quanzhou362000, China
    5Beijing Institute of Geothermal Research, Beijing100012, China

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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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