Constraining foraminiferal calcification depths in the western Pacific warm pool

Abstract

Insight into past changes of upper ocean stratification, circulation, and nutrient signatures rely on our knowledge of the apparent calcification depth (ACD) and ecology of planktonic foraminifera, which serve as archives for paleoceanographic relevant geochemical signals. The ACD of different species varies strongly between ocean basins, but also regionally. We constrained foraminiferal ACDs in the Western Pacific Warm Pool (Manihiki Plateau) by comparing stable oxygen and carbon isotopes (δ 18 O calite , δ 13 C calcite ) as well as Mg/Ca ratios from living planktonic foraminifera to in-situ physical and chemical water mass properties (temperature, salinity, δ 18 O seawater , δ 13 C DIC ). Our analyses point to Globigerinoides ruber as the shallowest dweller, followed by Globigerinoides sacculifer, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata and Globorotaloides hexagonus inhabiting increasing greater depths. These findings are consistent with other ocean basins; however, absolute ACDs differ from other studies. The uppermost mixed-layer species G. ruber and G. sacculifer denote mean calcification depths of~95 m and~120 m, respectively. These Western Pacific ACDs are much deeper than in most other studies and most likely relate to the thick surface mixed layer and the deep chlorophyll maximum in this region. Our results indicate that N. dutertrei appears to be influenced by mixing waters from the Pacific equatorial divergence, while P. obliquiloculata with an ACD of~160 m is more suitable for thermocline reconstructions. ACDs of G. hexagonus reveal a deep calcification depth of~450 m in oxygen-depleted, but nutrient-rich water masses, consistent to other studies. As the δ 13 C of G. hexagonus is in near-equilibrium with ambient seawater, we suggest this species is suitable for tracing nutrient conditions in equatorial water masses originating in extra-topical regions.

Stable Oxygen Isotopes and Mg/Ca in Planktic Foraminifera From Modern Surface Sediments of the Western Pacific Warm Pool: Implications for Thermocline Reconstructions

“…G. tumida calcifies within the transition between NPTW/SPTW and NPIW or AAIW. Our depth estimates generally agree with results from sediment traps and plankton tows in the central equatorial Pacific, North Pacific, and Indian Oceans (Kuroyanagi & Kawahata, ; Mohtadi et al, ; Peeters et al, ; Rippert et al, ; Watkins et al, ). However, our data show a tendency to somewhat deeper absolute calcification depths and wider depth ranges for most species, probably owing to a generally thick mixed layer and deep thermocline in the WPWP.…”

Modeling seasonal and vertical habitats of planktonic foraminifera on a global scale

“…In the transitional and subtropical waters, however, PLAFOM2.0 slightly underestimates the depth habitat of G. ruber (white) and T. sacculifer (Figs. 4d-e and 5d-e) as they inhabit the upper 125 m in the western North Atlantic and/or consistently occur from 0 to 200 m of water depth in the subtropical eastern North Atlantic (Rebotim et al, 2017) or in the seas surrounding Japan (Kuroyanagi and Kawahata, 2004). Nevertheless, both species typically live close to the surface (above 100 m) (e.g., Bé and Hamlin, 1967;Fairbanks et al, 1982;Kemle-von Mücke and Oberhänsli, 1999;Schiebel et al, 2002;Wilke et al, 2009;Rippert et al, 2016), thus being associated with a shallow depth habitat, which is reproduced by the model. Since T. sacculifer and G. ruber (white) are algal symbiont-bearing species, they are most abundant in the photic zone where light intensities are highest, but chlorophyll a concentrations and temperature also control their habitat.…”

“…However, at some locations both the model and observations show the reverse (see Fig. S4 and, e.g., Rippert et al, 2016;Rebotim et al, 2017), indicating that this depth ranking is not globally valid. In comparison with the temperate and cold-water species, G. ruber (white) and T. sacculifer are most abundant in the model in waters with temperatures above 22 • C and absent where temperature values drop below 15 • C (see Fig.…”

Stable water isotopes in the MITgcm

“…They limited their compilation to the four species, Globigerinoides ruber white (G. ruber (w)), Globigerina bulloides (G. bulloides), Neogloboquadrina pachyderma sinistral (N. pachyderma (s)) and Globigerinoides sacculifer (G. sacculifer), since these species are very abundant, cover a broad geographical and temporal range and belong to the shallowest-dwelling planktonic foraminifera. We extended this data set with available in situ δ 18 O c data from Kohfeld and Fairbanks (1996), Moos (2000), Stangeew (2001), Volkmann and Mensch (2001), Mortyn and Charles (2003), Keigwin et al (2005), Wilke et al (2009) and Rippert et al (2016). By using inverse distance weighting, we interpolated the δ 18 O c data to the nearest tracer grid point of the MITgcm grid (analogous to the GISS data) and compared them to the simulated long-term monthly mean δ 18 O c values of the respective month of sampling.…”