Further constraints on the diagenetic influences and salinity effect onGlobigerinoides ruber(white) Mg/Ca thermometry: Implications in the Mediterranean Sea

Abstract

[1] We analyzed Mg/Ca ratios of the planktonic species Globigerinoides ruber (white) picked from 49 box core samples covering the whole Mediterranean Sea and 2 core tops from the Atlantic Ocean. Over the entire data set, we found no significant correlation between Mg/Ca and d 18 O-derived calcification temperatures. This lack of correlation is chiefly due to the presence of an early diagenetic, Mg-rich calcite coating, which can constitute up to 20% of the total shell calcite in the central and eastern Mediterranean basin and result in anomalously high Mg/Ca values and a high scattering. In the western Mediterranean Sea, however, G. ruber Mg/Ca scattering shows smaller amplitude and Mg-rich calcite remains under the XRD detection limit. SEM observations indicate that only a few samples are affected by trace amounts of post-mortem calcite overgrowths (most of this calcite being likely removed during the chemical cleaning for Mg/Ca analyses). Using core top sediments from the western Mediterranean Sea, we performed an empirical Copyright 2011 by the American Geophysical Union 1 of 19 calibration exercise, which confirms that G. ruber Mg/Ca is not only related to temperature but it is also significantly affected by sea surface salinity. This salinity effect is not specific to high salinity environments such as the Mediterranean Sea, since it appears to be coherent with recent results obtained on Indo-Pacific and Atlantic surface sediments, which suggest that a +1 (psu) change in SSS results in a +1.7°C Mg/Ca-temperature bias. This sensitivity to salinity is significantly higher than those deduced from culture experiments.Components: 12,000 words, 7 figures, 3 tables.Keywords: Mediterranean Sea; early diagenetic calcite coating; foraminiferal Mg/Ca; salinity.

Assessing the reliability of foraminiferal Mg/Ca thermometry by comparing field-samples and culture experiments: a review

“…Nev er the less, there are also dis ad van tages and lim i ta tions, as this ap proach may be af fected by sec ond ary diagenetic effects, such as par tial dis so lu tion and over growth (Ta ble 1), which, at times, ren der dif fi cult the es tab lish ment of a re li able link be tween Mg/Ca and SST (e.g., Dekens et al, 2002;Regenberg et al, 2007Regenberg et al, , 2009Regenberg et al, , 2014Sadekov et al, 2010;Kontakiotis et al, 2011;Sabbatini et al, 2011;Fehrenbacher and Mar tin, 2014). For ex am ple, par tial dis so lu tion of foraminiferal cal cite tends to cause a de crease in Mg/Ca; since the sol u bil ity of car bon ate tends to in crease with depth at any lo ca tion, core-top ma te rial from greater depths will be par tic ularly prone to Mg/Ca al ter ation.…”

“…We pres ent an ex ten sive multi-for mat view (cul ture, core-top and plank ton tow sam ples) of the palaeoceanographically sig nif icant plank tonic foraminifer spe cies Globigerinoides ruber and (sub) spe cies-spe cific (in terms of both col our vari ants and distinct morphotypes in its white chromotype) re sponse to the S and over growth ef fects across a broad range of ocean o graphic re gions in the con text of pub lished cal i bra tion data, high light ing par tic u lar fea tures of each ap proach. We es pe cially fo cus on G. ruber since it seems to be the most ap pro pri ate spe cies for the S ef fect due to its con tin u ous pres ence oc cu py ing many overlap ping seg ments of the en tire S range, it is a sen si tive spe cies to ma nip u la tions dur ing cul tur ing ex per i ments (Hemleben et al, 1989;Kisakürek et al, 2008) as well to car bon ate diagenesis in high sa lin ity set tings (Kontakiotis et al, 2011;Boussetta et al, 2011;Sabbatini et al, 2011;Antonarakou et al, 2012), and because it is well in stru men tally val i dated .…”

“…This serves as a timely com ple ment to re cent field and culture stud ies, in which the po ten tial sig nif i cance of both S and diagenesis on Mg/Ca ra tios is dis cussed (e.g., Kisakürek et al, 2008;Fer gu son et al, 2008;Kontakiotis et al, 2011;Sabbatini et al, 2011;Hönisch et al, 2013;Antonarakou et al, 2015). We pres ent an ex ten sive multi-for mat view (cul ture, core-top and plank ton tow sam ples) of the palaeoceanographically sig nif icant plank tonic foraminifer spe cies Globigerinoides ruber and (sub) spe cies-spe cific (in terms of both col our vari ants and distinct morphotypes in its white chromotype) re sponse to the S and over growth ef fects across a broad range of ocean o graphic re gions in the con text of pub lished cal i bra tion data, high light ing par tic u lar fea tures of each ap proach.…”

“…Thus, the com piled data span a large range of S val ues, from 30.4 to 44.3 psu. In the case of the Med i ter ra nean Sea sam ples, we fur ther ac counted for diagenetic al ter ation, where pre sented, by ex clud ing all core-top sam ples sug gested to be im pacted by high-Mg overgrowths (Fer gu son et al, 2008;Kontakiotis et al, 2011;Sabbatini et al, 2011;Boussetta et al, 2011).…”

“…How ever, it has also been shown that the pro cess is con trolled by other en vi ron men tal pa ram e ters. Be cause foraminiferal Mg/Ca ra tios are con trolled by cal ci fi cation phys i ol ogy (Erez, 2003;Bentov and Erez, 2006), em pir i cal cal i bra tion stud ies are es sen tial to de ter mine the T sen si tiv ity, as well as the roles of any other con trib ut ing fac tors, such as salin ity (e.g., Nürnberg et al, 1996;Lea et al, 1999;Fer gu son et al, 2008;Kisakürek et al, 2008;Groeneveld et al, 2008;Duenas-Bohórquez et al, 2009;Hoogakker et al, 2009;Arbuszewski et al, 2010;Hönisch et al, 2013), car bon ate ion con cen tra tion (e.g., Rosenthal et al, 2006;Elderfield et al, 2006;Alison et al, 2010;Raitzsch et al, 2010), in or ganic diagenetic over print (e.g., Kontakiotis et al, 2011;van Raden et al, 2011;Boussetta et al, 2011;Sabbatini et al, 2011), and likely oth ers. It is nec es sary to pay close at ten tion on these envi ron men tal pa ram e ters, be cause in suf fi cient knowl edge of them can com pro mise use of foraminiferal Mg/Ca ra tios in confi dently re con struct ing past tem per a tures.…”