Fig 1.
Location of the study site in the northern equatorial Indian Ocean, the Maldives (blue star) and the comparative studies referenced in this work: A[13], B[16], C[14], D[12], E[17] and F[3] (world map from [18]).
Table 1.
Species list and associated data for geochemical analysis.
Fig 2.
Maps showing seasonal reversal of the South Asian Monsoon (SAM) winds and associated ocean currents during (a) winter and (b) summer and (c) the location of the study Site U1467 in the Maldives (blue star) and the measurement sites of the CTD profiles used in this study (yellow squares). WICC = West India Coastal Current, WMC = Winter Monsoon Current; SMC = Summer Monsoon Current; EICC = East India Coastal Current (c. modified after [20]).
Fig 3.
Conductivity, Temperature and Depth (CTD) probe data from the Maldives region, including summer (coarsely dashed and solid lines) and winter (finely dashed line) salinity, temperature, oxygen and fluorescence profiles [24,26,27].
SML = surface mixed layer.
Fig 4.
Plate illustrating light microscope and Scanning Election Microscope (SEM) images of the spiral (a, d), ventral (b, e) and umbilical view (c, f) for 1. Globoturborotalita rubescens (p), 2. Globigerinoides ruber (w), 3. Globigerinoides elongatus, 4. Globigerinoides pyramidalis, 5. Trilobatus sacculifer (w/s), 6. Globigerinella siphonifera, 7. Neogloboquadrina dutertrei. All scale bars = 100 μm, p = pink, w = white, w/s = with sac.
Fig 5.
Plate illustrating light microscope and Scanning Election Microscope (SEM) images of the spiral (a, d), ventral (b, e) and umbilical view (c, f) for 1. Globigerinita glutinata (w/b), 2. Globorotalia scitula, 3. Globorotaloides hexagonus, 4. Globigerina bulloides, 5. Globorotalia ungulata, 6. Globorotalia menardii, 7. Pulleniatina obliquiloculata (w/c), 8. Cibicides mabahethi. All scale bars = 100 μm, w/b = with bulla, w/c = with cortex.
Table 2.
Global studies used in this comparison (Fig 1).
Fig 6.
Mean δ18O and δ13C multi-species scatter plot with standard deviations (black bars) shown for all species.
Interpretations in grey after [54]. w = white; p = pink; w/s = with sac; w/b = with bulla; w/c = with cortex.
Table 3.
Raw δ18O, δ13C and Mg/Ca values for the 15 investigated species.
Fig 7.
Calculated vertical δ18Osw depth profiles derived using the salinity Eqs 5, 6 and 7 of [49], [47] and [50], respectively.
Summer (thin solid lines) and winter (thin dashed lines) salinity profiles of [26] and [27], respectively were used to show the seasonality and calculate overall averages (thick solid lines). The gridded δ18Osw data set by [51] for the Maldives region is shown for reference in grey. Black stars show mean measured δ18Osw values from the region for the surface (0 m) by [45–47], at 50 m by [55] and at ±500 m by [48] with the range in values represented by the grey shaded boxes. Equation numbers are identified on the graph in their respective colors.
Table 4.
Temperature calculations and seasonally averaged ACD estimates using different δ18Oc–temperature equations and an average δ18Osw value for all species (Method 1.1).
Table 5.
Temperature calculations and seasonally averaged ACD estimates using different δ18Oc−temperature equations and different δ18Osw values for the reported shallow-intermediate and deeper-dwelling species (Method 1.1).
Fig 8.
Calculated vertical δ18Oe depth profiles derived using δ18O-temperature Eqs 1, 2, 3 and 4 of [37], [38], [39] and [40], respectively.
All profiles were calculated using the δ18Osw profiles obtained from Eq 7 shown in Fig 7. Seasonal variations are represented by the thin lines (summer: solid lines; winter: dashed lines) with the thicker lines representing the overall averages. Equation numbers are identified on the graph in their respective colors.
Table 6.
Seasonally averaged isotope-ACDs assigned using δ18Oe curves calculated using different δ18Oc−temperature Eqs 1–4 and species-specific Eqs. in conjunction with the generated δ18Osw curves from Eq 7 [50], (Method 1.2).
Fig 9.
Range in temperature calculations using various species-specific Mg/Ca-temperature equations for six planktonic and one benthic species.
GRW: G. ruber (w), TS: T. sacculifer (w/s), GS: G. siphonifera, GB: G. bulloides, PO: P. obliquiloculata (w/c), GM: G. menardii & CM/W: C. mabahethi/wuellerstorfi with w = white, w/s = with sac, w/c = with cortex, grey shading indicates temperature (T) exclusion zones as outlined in the methods section. Horizontal black lines indicate the selected equation for each species. See Table 7 for species-specific equation references which are also indicated by a * below. (Planktonic references a: [59], b: [60]*, c: [61], d: [62], e: [63], f: [64], g: [65], h: [13]*, i: [66], j: [67], k: [68], l: [69]*, m: [70]; Benthic references n: [71], o: [52], p: [72], q: [73], r: [74], s: [53], t: [75], u: [76]*, v: [77]. See S2 Table for more information regarding each equation.
Table 7.
Assigned, seasonally averaged Mg/Ca-ACDs (Method 2).
Fig 10.
(a) Comparison of the ACDs calculated from this study (blue star: blue blocks denote the range in isotope-ACDs averaged from the best options from both Methods 1.1 and 1.2 and dark blue bars show the Mg/Ca-ACDs obtained from Method 2) and the five (sub)tropical global studies (black blocks) with (b) showing the vertical thermal structure through the water column at each study location. Grey text denotes the size of the tests used for each study. The blue dashed lines indicate the average isotope-ACDs for this study. Data sources: A [13]; B [16]; C [14]; D [12] and E [17] with only the west Atlantic (W-Atlantic) derived ACDs shown for study B. SML = surface mixed layer, w = white, p = pink, w/s = with sac, w/o = without, w/b = with bulla, w/c = with cortex.
Fig 11.
Average isotope-ACDs overlain on the modern day summer (coarsely dashed and solid line) and winter (finely dashed line) CTD data of [24], [26] and [27], respectively.
These seasonal CTD datasets were used to show the seasonality and calculate the overall CTD averages. Standard deviations are represented by the colored bars for each species with reference to the regional fluorescence and seawater density profiles also given. DCM: deep chlorophyll maximum with fluorescence peaks F1, F2 and F3; w = white; p = pink; w/s = with sac; w/b = with bulla; w/c = with cortex.
Fig 12.
Comparison of the ACDs calculated from this study (Blue blocks: average of the best isotope-ACDs for Method 1.1 and 1.2; dark blue bars denote the Mg/Ca-ACD estimates from Method 2) with the adult ALD estimates from the plankton tows of [3], with their non-upwelling stations represented in black and upwelling stations represented in grey.