The late Pliocene Benguela upwelling status revisited by means of multiple temperature proxies

Abstract

[1] As compared to the late Pleistocene, Alkenone-based sea surface temperature (SST) in the Benguela region revealed relatively warm and stable SST recorded between 3.5 and 2.0 Ma, and coincide with a period of increasing biological productivity as revealed by increasing deposition of biogenic opal. We assess how the hydrological patterns recorded in SST proxies are embedded in the geological record by performing a proxy-proxy comparison. We used Laser-Ablation Inductively Coupled Plasma-Mass Spectrometry to measure the Mg/Ca on the planktonic foraminifera species Globigerina bulloides, allowing in situ measurements of Mg/Ca on individual foraminiferal tests. Mg/Ca-derived temperatures provide much colder temperatures than alkenone-derived SST by up to 10 C. We build a scenario involving contrasting sensitivities of paleothermometers upon the annual cycle, namely alkenones preferentially capturing SST when the surface ocean is warmer than the mean-annual average SST, and G. bulloides capturing SST when upwelling intensifies. Multichamber analysis also suggests that G. bulloides migrates below the sea surface while calcifying its last chambers prior to gametogenesis, allowing the extraction of both surface and subsurface temperature from Mg/Ca measured on different chambers. The range of temperatures recorded between our multiple SST proxies is supported by the range of temperatures simulated with a general circulation model when different seasons, different water depth and different orbital configurations occurring during the late Pliocene are considered. A greater seasonal cycle in SST during the Pliocene can account for alkenone and Mg/Ca-derived temperature contrast, pointing to a radically different mode of upwelling activity in the Benguela region compared to today.

Mn∕Ca intra- and inter-test variability in the benthic foraminifer <i>Ammonia tepida</i>

“…Secondly, each laser ablation profile was screened for peaks in elements that may indicate surface contamination ( 27 Al, 57 Fe, 66 Zn). Typically, high concentrations of Mn on the outer and inner shell surfaces are considered an indicator of contamination (Marr et al, 2011;de Nooijer et al, 2014a;Leduc et al, 2014;Koho et al, 2015). In our case, where Mn is the element of interest, whenever peaks of Mn on outer and inner parts of profiles corresponded to peaks of other contaminant elements, they were discarded from our data.…”

Growth of the Maritime Continent and its possible contribution to recurring Ice Ages

“…Alkenones, TEX 86 , and Mg/Ca ratios have been used to infer changes in ocean temperatures above the thermocline; widespread and monotonic cooling has been found in the eastern tropical Pacific [ Dekens et al ., ; Groeneveld et al ., ; Lawrence et al ., ; Wara et al ., ; Y. G. Zhang et al ., 2014], off the coast of Peru [ Dekens et al ., ], off Southern California [ Dekens et al ., ], in the Caribbean [ O ' Brien et al ., ], in the North Atlantic [ Lawrence et al ., ], off the west coasts of northern Africa [ Herbert and Schuffert , ] and southern Africa [ Etourneau et al ., ; Marlow et al ., ; Rosell‐Melé et al ., ] (though for this last case, Leduc et al . [] suggest that cooling applies only to the warm season), in the southeastern Indian Ocean [ Karas et al ., ], and in the South China Sea [ O ' Brien et al ., ]. In the western equatorial Pacific and eastern Indian Ocean, long‐term cooling is more ambiguous.…”

Nano‐Powdered Calcium Carbonate Reference Materials: Significant Progress for Microanalysis?

“…Some of them are very small in the 10–100 µm range and/or have closely spaced compositionally different layers. Laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) has become a suitable technique for microanalysis of these materials (e.g., Wassenburg et al , Jochum et al , Jentzen et al , Leduc et al , Yang et al , Mertz‐Kraus et al , Hathorne et al . , Vetter et al , Caragnano et al , Schiebel and Hemleben , Weber et al ), but well‐characterised homogeneous reference materials (RMs) for calibration at the nanometre to micrometre scale are needed.…”

“…Some of them are very small in the 10-100 µm range and/or have closely spaced compositionally different layers. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) has become a suitable technique for microanalysis of these materials (e.g., Wassenburg et al 2016b, Jentzen et al 2018, Leduc et al 2014, Yang et al 2014, Mertz-Kraus et al 2009, Hathorne et al 2009, Vetter et al 2013, Caragnano et al 2014, Schiebel and Hemleben 2017, Weber et al 2018a), but well-characterised homogeneous reference materials (RMs) for calibration at the nanometre to micrometre scale are needed. For such purposes, different kinds of RMs were used, for example, the silicate glasses from the National Institute of Standards and Technology (NIST), geological glasses from the United States Geological Survey (USGS), the Max Planck Institute for Chemistry (MPI-DING) and synthetic or natural carbonates (USGS) (Jochum and Enzweiler 2014).…”