instrumentally observed climate variability and may validate global circulation models used to predict future climate change. Most promising in this respect is the
Mg/Ca paleothermometry, which is based on the temperature dependence of the substitution of magnesium into
biogenic calcite. It has long been known that tropical calcitic shells of marine organisms are generally more
enriched in magnesium than subpolar shells (e.g., Savin
and Douglas, 1973). But in particular for the important
group of foraminifera (marine protozoa), a well-defined
and species-specific calibration of the shell Mg/Ca ratio
on ocean temperature was missing until the mid-1990s
(Nürnberg, 1995; Nürnberg et al., 1996). Such speciesspecific Mg/Ca versus temperature relationships, however, are essential for all further quantitative paleotemperature reconstructions.
Meanwhile, the analytical protocol for Mg analyses has
established (Greaves et al., 2008), and the Mg/Ca
paleothermometry providing an SST accuracy of
~0.5–1
C became one of the most important tools in
paleoceanography. From cultivation experiments and
core-top calibration studies, an exponential relationship
between temperature and foraminiferal Mg/Ca was
suggested for most planktonic species (e.g., Elderfield
and Ganssen, 2000; Anand et al., 2003; Regenberg et al.,
2009). Although temperature is the primary control on
Mg/Ca, salinity and pH exert additional influence on
Mg
2+ uptake. Mg/Ca positively changes with salinity
(Nürnberg et al., 1996; Kisakürek et al., 2008), which
requires caution when expecting large-scale salinity variations through time. Seawater pH, instead, is negatively
correlated to Mg
2+ incorporation (Kisakürek et al.,
2008). Both effects are assumed to cancel each other
(Lea et al., 2000).
The method offers unique advantages compared with
other SST proxies. Most importantly, Mg/Ca is measured on the same biotic carrier as stable oxygen isotopes (d
18 O), thereby avoiding the seasonality and/or
habitat effects that occur when proxy data from different faunal groups are used. Also, the magnitude and
timing of SST and d
18 O changes can be separated,
allowing to constrain the timing of surface ocean
warming with respect to continental ice sheet melting
and to approximate changes in ocean salinity (e.g.,
Nürnberg, 2000).
Most prejudicial to Mg/Ca paleothermometry is that
calcite dissolution may cause the preferential removal of
relatively soluble magnesium-enriched calcitic parts of
the shells (Brown and Elderfield, 1996) and thus may limit
the applicability of Mg/Ca as a tracer for SST. Regenberg
et al. (2006) defined the oceanic 18–26 mm kg
À1 DCO 3
2À
threshold of calcite saturation as critical for the removal.
Various algorithms to correct for dissolution effects were
proposed accordingly (Rosenthal et al., 2000; Dekens
et al., 2002; Regenberg et al., 2006). Whether the longterm changes in seawater Mg/Ca (Tyrrell and Zeebe,
2004) need to be considered for the Mg/Ca
paleothermometry, it is still deficiently studied.
Bibliography
Anand, P., Elderfield, H., and Comte, M. H., 2003. Calibration of
Mg/Ca thermometry in planktonic foraminifera from a sediment
trap time series. Paleoceanography, 18, PA1050, doi:10.1029/
2002PA000846.
Brown, S. J., and Elderfield, H., 1996. Variations in Mg/Ca and
Sr/Ca ratios of planktonic foraminifera caused by postdepositional dissolution: evidence of shallow Mg-dependent dissolution. Paleoceanography, 11(5), 543–551.
Dekens, P. S., Lea, D. W., Pak, D. K., and Spero, H. J., 2002. Core
top calibration of Mg/Ca in tropical foraminifera: refining
paleotemperature estimation. Geochemistry, Geophysics,
Geosystems, 3, 1–29, doi:10.1029/2001GC00.
Elderfield, H., and Ganssen, G., 2000. Past temperature and d
18 O of
surface ocean waters inferred from foraminiferal Mg/Ca ratios.
Nature, 405, 442–445.
Greaves, M., Caillon, N., Rebaubier, H., Bartoli, G., Bohaty, S.,
Cacho, I., Clarke, L., Cooper, M., Daunt, C., Delaney, M.,
deMenocal, P., Dutton, A., Eggins, S., Elderfield, H., Garbe-Schönberg, C. D., Goddard, E., Green, D., Groeneveld, J., Hastings,
D., Hathorne, E., Kimoto, N., Klinkhammer, G., Labeyrie, L.,
Lea, D. W., Marchitto, T., Martínez-Botí, M. A., Mortyn, P. G.,
Ni, Y., Nürnberg, D., Paradis, G., Pena, L., Quinn, T., Rosenthal,
Y., Russell, A., Sagawa, T., Sosdian, S., Stott, L., Tachikawa, K.,
Tappa, E., Thunell, R., and Wilson, P. A., 2008. Interlaboratory
comparison study of calibration standards for foraminiferal
Mg/Ca thermometry. Geochemistry, Geophysics, Geosystems,
9(8), 1–27, doi:10.1029/2008GC001974.
Kisakürek, B., Eisenhauer, A., Böhm, F., Garbe-Schönberg, D., and
Erez, J., 2008. Controls on shell Mg/Ca and Sr/Ca in cultured
planktonic foraminiferan, Globigerinoides ruber (white). Earth
and Planetary Science Letters, 273, 260–269, doi:10.1016/j.
epsl.2008.06.026.
Lea, D. W., Pak, D. K., and Spero, H. W., 2000. Climate impact of
later Quaternary equatorial Pacific sea surface temperature variations. Science, 289, 1719–1724.
Nürnberg, D., 1995. Magnesium in tests of Neogloboquadrina
pachyderma sinistral from high northern and southern latitudes.
Journal of Foraminiferal Research, 25, 350–368.
Nürnberg, D., 2000. Taking the temperature of past ocean surfaces.
Science, 289, 1698–1699.
Nürnberg, D., Bijma, J., and Hemleben, C., 1996. Assessing the
reliability of magnesium in foraminiferal calcite as a proxy for
water mass temperatures. Geochimica et Cosmochimica Acta,
60, 803–814.
Regenberg, M., Nürnberg, D., Steph, S., Groeneveld, J., Garbe-Schönberg, D., Tiedemann, R., and Dullo, W. C., 2006. Assessing
the effect of dissolution on planktonic foraminiferal Mg/Ca
ratios: evidence from Caribbean core tops. Geochemistry,
Geophysics,
Geosystems,
7,
1–23,
doi:10.1029/
2005GC001019002E.
Regenberg, M., Steph, S., Nürnberg, D., Tiedemann, R., and GarbeSchönberg, D., 2009. Calibrating Mg/Ca ratios of multiple
planktonic foraminiferal species with d
18 O-calcification temperatures: paleothermometry for the upper water column. Earth and
Planetary Science Letters, 278, 324–336, doi:10.1016/j.
epsl.2008.12.019.
Rosenthal, Y., Lohmann, G. P., Lohmann, K. C., and Sherrell,
R. M., 2000. Incorporation and preservation of Mg in
Globigerinoides sacculifer: implications for reconstructing the
temperature and
18 O/
16 O of seawater. Paleoceanography, 15,
135–145.
Savin, S. M., and Douglas, R. G., 1973. Stable isotope and magnesium geochemistry of recent planktonic foraminifera from the
South Pacific. Geological Society of America Bulletin, 84,
2327–2342.
500
Mg/Ca PALEOTHERMOMETRY
Mg/Ca paleothermometry, which is based on the temperature dependence of the substitution of magnesium into
biogenic calcite. It has long been known that tropical calcitic shells of marine organisms are generally more
enriched in magnesium than subpolar shells (e.g., Savin
and Douglas, 1973). But in particular for the important
group of foraminifera (marine protozoa), a well-defined
and species-specific calibration of the shell Mg/Ca ratio
on ocean temperature was missing until the mid-1990s
(Nürnberg, 1995; Nürnberg et al., 1996). Such speciesspecific Mg/Ca versus temperature relationships, however, are essential for all further quantitative paleotemperature reconstructions.
Meanwhile, the analytical protocol for Mg analyses has
established (Greaves et al., 2008), and the Mg/Ca
paleothermometry providing an SST accuracy of
~0.5–1
C became one of the most important tools in
paleoceanography. From cultivation experiments and
core-top calibration studies, an exponential relationship
between temperature and foraminiferal Mg/Ca was
suggested for most planktonic species (e.g., Elderfield
and Ganssen, 2000; Anand et al., 2003; Regenberg et al.,
2009). Although temperature is the primary control on
Mg/Ca, salinity and pH exert additional influence on
Mg
2+ uptake. Mg/Ca positively changes with salinity
(Nürnberg et al., 1996; Kisakürek et al., 2008), which
requires caution when expecting large-scale salinity variations through time. Seawater pH, instead, is negatively
correlated to Mg
2+ incorporation (Kisakürek et al.,
2008). Both effects are assumed to cancel each other
(Lea et al., 2000).
The method offers unique advantages compared with
other SST proxies. Most importantly, Mg/Ca is measured on the same biotic carrier as stable oxygen isotopes (d
18 O), thereby avoiding the seasonality and/or
habitat effects that occur when proxy data from different faunal groups are used. Also, the magnitude and
timing of SST and d
18 O changes can be separated,
allowing to constrain the timing of surface ocean
warming with respect to continental ice sheet melting
and to approximate changes in ocean salinity (e.g.,
Nürnberg, 2000).
Most prejudicial to Mg/Ca paleothermometry is that
calcite dissolution may cause the preferential removal of
relatively soluble magnesium-enriched calcitic parts of
the shells (Brown and Elderfield, 1996) and thus may limit
the applicability of Mg/Ca as a tracer for SST. Regenberg
et al. (2006) defined the oceanic 18–26 mm kg
À1 DCO 3
2À
threshold of calcite saturation as critical for the removal.
Various algorithms to correct for dissolution effects were
proposed accordingly (Rosenthal et al., 2000; Dekens
et al., 2002; Regenberg et al., 2006). Whether the longterm changes in seawater Mg/Ca (Tyrrell and Zeebe,
2004) need to be considered for the Mg/Ca
paleothermometry, it is still deficiently studied.
Bibliography
Anand, P., Elderfield, H., and Comte, M. H., 2003. Calibration of
Mg/Ca thermometry in planktonic foraminifera from a sediment
trap time series. Paleoceanography, 18, PA1050, doi:10.1029/
2002PA000846.
Brown, S. J., and Elderfield, H., 1996. Variations in Mg/Ca and
Sr/Ca ratios of planktonic foraminifera caused by postdepositional dissolution: evidence of shallow Mg-dependent dissolution. Paleoceanography, 11(5), 543–551.
Dekens, P. S., Lea, D. W., Pak, D. K., and Spero, H. J., 2002. Core
top calibration of Mg/Ca in tropical foraminifera: refining
paleotemperature estimation. Geochemistry, Geophysics,
Geosystems, 3, 1–29, doi:10.1029/2001GC00.
Elderfield, H., and Ganssen, G., 2000. Past temperature and d
18 O of
surface ocean waters inferred from foraminiferal Mg/Ca ratios.
Nature, 405, 442–445.
Greaves, M., Caillon, N., Rebaubier, H., Bartoli, G., Bohaty, S.,
Cacho, I., Clarke, L., Cooper, M., Daunt, C., Delaney, M.,
deMenocal, P., Dutton, A., Eggins, S., Elderfield, H., Garbe-Schönberg, C. D., Goddard, E., Green, D., Groeneveld, J., Hastings,
D., Hathorne, E., Kimoto, N., Klinkhammer, G., Labeyrie, L.,
Lea, D. W., Marchitto, T., Martínez-Botí, M. A., Mortyn, P. G.,
Ni, Y., Nürnberg, D., Paradis, G., Pena, L., Quinn, T., Rosenthal,
Y., Russell, A., Sagawa, T., Sosdian, S., Stott, L., Tachikawa, K.,
Tappa, E., Thunell, R., and Wilson, P. A., 2008. Interlaboratory
comparison study of calibration standards for foraminiferal
Mg/Ca thermometry. Geochemistry, Geophysics, Geosystems,
9(8), 1–27, doi:10.1029/2008GC001974.
Kisakürek, B., Eisenhauer, A., Böhm, F., Garbe-Schönberg, D., and
Erez, J., 2008. Controls on shell Mg/Ca and Sr/Ca in cultured
planktonic foraminiferan, Globigerinoides ruber (white). Earth
and Planetary Science Letters, 273, 260–269, doi:10.1016/j.
epsl.2008.06.026.
Lea, D. W., Pak, D. K., and Spero, H. W., 2000. Climate impact of
later Quaternary equatorial Pacific sea surface temperature variations. Science, 289, 1719–1724.
Nürnberg, D., 1995. Magnesium in tests of Neogloboquadrina
pachyderma sinistral from high northern and southern latitudes.
Journal of Foraminiferal Research, 25, 350–368.
Nürnberg, D., 2000. Taking the temperature of past ocean surfaces.
Science, 289, 1698–1699.
Nürnberg, D., Bijma, J., and Hemleben, C., 1996. Assessing the
reliability of magnesium in foraminiferal calcite as a proxy for
water mass temperatures. Geochimica et Cosmochimica Acta,
60, 803–814.
Regenberg, M., Nürnberg, D., Steph, S., Groeneveld, J., Garbe-Schönberg, D., Tiedemann, R., and Dullo, W. C., 2006. Assessing
the effect of dissolution on planktonic foraminiferal Mg/Ca
ratios: evidence from Caribbean core tops. Geochemistry,
Geophysics,
Geosystems,
7,
1–23,
doi:10.1029/
2005GC001019002E.
Regenberg, M., Steph, S., Nürnberg, D., Tiedemann, R., and GarbeSchönberg, D., 2009. Calibrating Mg/Ca ratios of multiple
planktonic foraminiferal species with d
18 O-calcification temperatures: paleothermometry for the upper water column. Earth and
Planetary Science Letters, 278, 324–336, doi:10.1016/j.
epsl.2008.12.019.
Rosenthal, Y., Lohmann, G. P., Lohmann, K. C., and Sherrell,
R. M., 2000. Incorporation and preservation of Mg in
Globigerinoides sacculifer: implications for reconstructing the
temperature and
18 O/
16 O of seawater. Paleoceanography, 15,
135–145.
Savin, S. M., and Douglas, R. G., 1973. Stable isotope and magnesium geochemistry of recent planktonic foraminifera from the
South Pacific. Geological Society of America Bulletin, 84,
2327–2342.
500
Mg/Ca PALEOTHERMOMETRY