Δ18O and Mg/Ca Thermometry in Planktonic Foraminifera: A Multiproxy Approach Toward Tracing Coastal Upwelling Dynamics. Issue 2 (20th February 2020)

• Record Type: Journal Article
• Authors: Salgueiro, E.; Voelker, A. H. L.; Martin, P. A.; Rodrigues, T.; Zúñiga, D.; Froján, M.; de la Granda, F.; Villacieros‐Robineau, N.; Alonso‐Pérez, F.; Alberto, A.; Rebotim, A.; González‐Álvarez, R.; Castro, C. G.; Abrantes, F.
• Is Part Of: Paleoceanography and paleoclimatology. Volume 35:Issue 2(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-02-20
• Subjects: Mg/Ca -- δ18O -- Alkenone Uk'37 index -- planktonic foraminifera -- seasonal upwelling -- plankton net; Paleoceanography -- Periodicals; Paleoclimatology -- Periodicals; 551.46
• DOI: 10.1029/2019PA003726
• Languages: English
• ISSNs: 2572-4517

Abstract

Abstract: Planktonic foraminifera δ 18 O and Mg/Ca ratios are widely considered as a powerful proxy to reconstruct past seawater‐column temperature. Due to the complex interpretation of planktonic foraminifera δ 18 O data in regard to past seawater temperatures, temperature determination based on the foraminifera shell Mg/Ca ratio is believed to be more accurate. Scarce Mg/Ca calibration data exists for coastal upwelling regions, resulting in incoherent results of past seawater reconstructions. The current study along the NW Iberia coastal upwelling system intends to define the best Mg/Ca temperature equation for the most representative species of this region ( Neogloboquadrina incompta, Globigerina bulloides, and Globorotalia inflata ). Seawater temperature from δ 18 O and Mg/Ca of these three planktonic foraminifera species was compared with the surface sediments alkenone derived SST and with the in situ temperatures measured at the depths where these foraminifera species currently live and calcify. The equations that better reflect each species calcification depth were selected as our regional equations for δ 18 O and Mg/Ca temperature reconstructions. The δ 18 O‐estimated temperatures for surface sediment specimens were comparable with in situ seawater‐column temperature measurements, whereas the Mg/Ca derived temperatures seem to underestimate in situ values, in special for G. bulloides from samples affected by stronger coastal upwelling. The G. bulloides δ 18 O and Mg/Ca estimated temperatures from samples located offshore, further from coastal upwelling influence, are comparable to surface sediment alkenone derived temperatures. Our study shows that in upwelling areas, regional calibration of planktonic foraminifera Mg/Ca temperature equations is necessary for reliable interpretations of high‐resolution past temperature variability in these important environments. Plain Language Summary: To reconstruct past seawater temperatures from foraminifera shells' Mg/Ca content, implies the use of calibration equations. Most of the existing equations do not include samples from coastal upwelling areas. In this work, we have calibrated both δ 18 O and Mg/Ca measured in the planktonic foraminifer's dominant species in surface sediments from NW Iberian margin. We used in situ temperatures, at the depths where these foraminiferal species currently live and calcify, and the alkenone‐derived temperature for the same sediments. δ 18 O estimated temperatures are equivalent to in situ seawater column temperatures, while Mg/Ca derived temperatures seem to underestimate in situ values, in special for G. bulloides in samples close to the coast, under stronger coastal upwelling influence. However, outside upwelling influence results are comparable to alkenone‐derived temperatures. Our study shows that to estimate past temperatures from planktonic foraminifera Mg/Ca, with confidence, regional calibrations are necessary, in particular for coastal upwelling systems. Key Points: To define the appropriate δ 18 O and Mg/Ca paleotemperature equations to be used in the most abundant planktonic foraminifera species preserved in the ocean floor at the NW Iberia coastal upwelling system δ 18 O‐temperatures mirror in situ temperatures while the Mg/Ca‐temperatures underestimate the observed values, especially for G. bulloides, under stronger coastal upwelling influence G. bulloides δ 18 O and Mg/Ca estimated temperatures are comparable to surface sediment alkenone‐temperatures for samples located offshore, without a strong coastal upwelling