Resumen de Validation and application of planktonic foraminiferal-based geochemical proxies for climate reconstruction: end water mass tracer and mg/ca paleothermometer
This Ph.D. dissertation focuses on the validation of planktonic foraminiferal-based Mg/Ca and eNd proxies by means of plankton tow samples and their application to the reconstruction of oceanic parameters during the last deglaciation in the Eastern Equatorial Pacific.
We present water-column plankton-tow Mg/Ca results from 4 different areas (N Atlantic, SE Atlantic, NE Pacific and Norwegian Sea) and for 8 species (both globorotaliids and non-globorotaliids) and compare them to available core-tops and calibration equations. Some species (i.e., N. incompta, O. universa, G. ruber, G. sacculifer and G. truncatulinoides) present Mg/Ca ratios in agreement with published equations, while others have elevated and scattered Mg/Ca ratios (G. bulloides, G. inflata, G. hirsuta). One possible factor influencing the proxy is the difference between collection temperature and the temperature experienced during foraminiferal life cycle, with part of the tests being calcified in shallower and warmer waters. Furthermore, plankton tow foraminifera can be collected before the completion of their life cycle, implying that the test is not completely calcified and that characteristic low-Mg parts are missing. In addition, N. incompta Mg/Ca ratios are found to be correlated with salinity, highlighting the importance of this parameter besides temperature.
Nd/Ca ratios have been measured in an extensive and multi-specific plankton tow sample set from several geographic locations (SE Atlantic, NE Atlantic, Norwegian Sea, W Mediterranean), together with core-top samples from the Mediterranean region. The range of Nd/Ca ratios in plankton-towed foraminifera is similar to previously published analyses of sedimentary foraminifera. For the Mediterranean, where core-top and plankton tow data are both available, the range for plankton tows is essentially identical to that for the core-tops. Re-adsorption of Nd during cleaning is ruled out by the fact that the plankton tow samples underwent only an oxidative cleaning process. There is a relationship between Mn and Nd in plankton-tow samples that is mirrored by a similar correlation in core-top samples, suggesting that Fe-Mn coatings are of negligible importance to the Nd contents of foraminifera as the Nd/Mn ratio it implies is over an order of magnitude greater than that seen in other Fe-Mn oxide phases. Rather, since both plankton-tows and core-tops present a similar behaviour, the Nd/Mn relationship must originate in the upper water column. The data are consistent with the acquisition of Nd and Mn from the water column by binding to organic material. Collectively, the results explain the high rare earth element contents in sedimentary foraminifera and the fact that they record a surface water Nd isotopic signal.
The Holocene and Last Glacial Maximum Nd isotopic variability of the ODP Leg 202 study area (Equatorial/South Eastern Pacific) have been characterized. Both detrital and foraminiferal eNd correlate well with Central and South American volcanic sources immediately to the east/south-east, in agreement with the predominant south/south-easterly trade winds pattern. Foraminiferal eNd does not display a close relationship with seawater eNd, which suggests that continental contributions mainly determine the Nd isotopic composition of the Pacific Ocean in this region. The deglaciation history of Site 1238 has been investigated with a multi-proxy approach. During the deglaciation, the Equatorial Undercurrent (EUC) experienced episodes of increased strength due to enhanced upwelling of Circumpolar Deep Water and subsequent advection to the Subantarctic Mode Water formation sites. The EUC brought thermal and geochemical anomalies to the Eastern Equatorial Pacific, namely an increase in water temperatures, a decrease in d13C and a more radiogenic eNd signal. The Neogloboquadrina dutertrei eNd record has revealed that the Nd isotopic composition of the water column at Site 1238 is mainly controlled by the eolian input of continental dust and the lateral advection of EUC waters. The detrital eNd record shows a change in continental sources between the Last Glacial Maximum/deglaciation and the Holocene, probably due to increased trade winds during the former period.
Overall, the results of this dissertation place the use of the Mg/Ca paleothermometer and the eNd water mass tracer on a sure footing and apply them to the reconstruction of the events that occurred during the deglaciation in the Eastern Equatorial Pacific.
