A novel approach to dissolution correction of Mg/Ca–based paleothermometry in the tropical Pacific

Abstract

[1] Mg/Ca in planktonic foraminifers carries two main signals: calcification temperature and postdepositional test dissolution. Shell dissolution thus distorts water temperature reconstructions made with Mg/Ca in foraminifers. This problem could be resolved by quantifying the impact of carbonate dissolution on Mg/Ca with an independent, temperature-insensitive deep-sea calcite dissolution proxy, such as the Globorotalia menardii fragmentation index (MFI). To test the validity of this approach, we measured Mg/Ca in the tests of several planktonic foraminifers and MFI in core tops collected over a wide geographic region of the tropical Pacific and covering a wide range of deep-sea calcite dissolution and seawater temperature. We confirm that Mg/Ca from different species have different susceptibility to temperature and dissolution. Mg/Ca in surface-dwelling Globigerina bulloides is controlled by calcification temperature and is largely unaffected by carbonate dissolution estimated from MFI. In contrast, Mg/Ca in deeper dwelling G. menardii is minimally sensitive to temperature and dominantly affected by dissolution. Mg/Ca in Neogloboquadrina dutertrei and Pulleniatina obliquiloculata are significantly affected by both temperature and dissolution, and MFI can be effectively used to correct temperature estimates from these species for calcite dissolution. Additional variables besides temperature and dissolution appear to control Mg/Ca in Globorotalia tumida, and their identification is a prerequisite for interpreting elemental shell composition in this species. Combining down-core measurements of Mg/Ca in multiple foraminifer species with MFI provides a powerful tool for reconstructing past changes in the upper water column temperature structure in the tropical Pacific.

Holocene temperature evolution of the subpolar North Atlantic recorded in the Mg/Ca ratios of surface and thermocline dwelling planktonic foraminifers

“…In the eastern North Atlantic Schiebel et al (1997) restricted this to the upper 60 m -consistent with pump and plankton tow data from our study area in July-August 1994 (M. R. Chapman, unpublished data). Mg/Ca studies of G. bulloides suggested that the best relationship between G. bulloides Mg/Ca ratios and temperature is at 30 m water depth (Mekik et al, 2007), further confirming the near surface preference of this species. The present day hydrography at the core site (SST range of~8°C in February to~12°C in August) match closely the idealised temperature window for this spring-summer species, suggesting that, unless maximum temperatures fall below the 8°C threshold, we are unlikely to have any problems with periods of reduced or no productivity over the Holocene sequence of our record.…”

“…Mekik et al (2007) suggest that surface dwelling species, and G. bulloides in particular, are far less susceptible to such effects and exhibit no significant dissolution. They concluded that G. bulloides was therefore a species that had the potential to provide a pure temperature signal in its Mg/Ca ratio.…”

A strong temperature effect on U/Ca in planktonic foraminiferal carbonates

“…U/Ca and Mg/Ca in G. sacculifer are also decreased (by $30% and $18%, respectively) at Ceara Rise, but changes at OJP are less clear for this species (Russell et al, 1994;Brown and Elderfield, 1996). The decrease in element/Ca ratios is probably caused by heterogeneous distributions of elements in shells and preferential dissolution of shells and parts enriched with these elements in under-saturated deep waters (Russell et al, 1994;Brown and Elderfield, 1996;Rosenthal et al, 1997;Fehrenbacher et al, 2006;Mekik and Franc ßois, 2006;Mekik et al, 2007). In this study, the two dissolution affected samples are not included in Figs.…”

Holocene and Eemian sea surface temperature trends as revealed by alkenone and Mg/Ca paleothermometry

“…Also, differential partial planktonic foraminifera test dissolution can have modulated the amplitude of SST changes in this region, which is known to be particularly corrosive (Mekik et al, 2007). For example, an increase in primary productivity over the Holocene would necessarily have increased biologically-mediated calcite dissolution over this time period, potentially leading to decreased Mg/Ca-derived SST estimations.…”