Ca Paleothermometry from Calcareous Marine Fossils

Mg/Ca and Sr/Ca Paleothermometry from Calcareous Marine Fossils Y Rosenthal, Rutgers University, New Brunswick, NJ, USA B Linsley, Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA ã 2013 Elsevier B.V. All rights reserved.

Introduction In the ocean, the distribution of sea surface and bottom water temperature (BWT) and salinity is perhaps the best representation of the state of the climate system, as the ocean plays a fundamental role in the evolution of Earth’s climate. Therefore, determining the past temperature evolution of the ocean is key for understanding Earth’s climate history. Among the most often used elemental proxies of paleotemperature are Mg/Ca in planktonic Foraminifera and Sr/Ca in corals as recorders of sea surface temperature (SST) and Mg/Ca in benthic Foraminifera and ostracodes as proxies of BWT. Mg, Sr, and Ca have relatively long oceanic residence times, about 13 My for Mg, 5 My for Sr, and 1 My for Ca, implying nearly constant Mg-, and Sr- to- Ca ratios in seawater on timescales of less than a million years. A clear advantage of these carbonate-based thermometers is that coupling d18O and Mg/Ca measurements in Foraminifera, or Sr/Ca in corals, potentially provides a novel way to adjust for the temperature-dependency of d18O in CaCO3 minerals and isolate the record of seawater d18O (d18Oseawater; Mashiotta et al., 1999), which can then be used to reconstruct local changes in evaporation–precipitation (and by inference salinity; e.g., Lea et al., 2000), advection of water with different seawater oxygen isotopic composition (d18Oseawater) and provide valuable information about changes in continental ice volume (e.g., Lear et al., 2000).

Thermodynamic Effects on Mg and Sr Coprecipitation in Carbonates Among the alkaline earth elements, Mg2þ and Sr2þ form the most important solid solutions with carbonates. Because of the difference in the ionic radii between Sr2þ (large) and Mg2þ (small), SrCO3 is isostructural with the aragonite crystal cell (orthorombic), whereas MgCO3 is isostructural with calcite (rhombohederal). The Mg/Ca and Sr/Ca ratios in CaCO3 minerals depend on two factors: the Mg/Ca and Sr/Ca activity ratios of ocean water, and the distribution coefficients of Mg/ Ca and Sr/Ca between calcite, aragonite, and seawater, respectively. These relationships are expressed as DEl ¼ (El/Ca)mineral/ (El/Ca)seawater, where DEl, is the empirical homogeneous distribution coefficient calculated based on the molar concentration ratios of Mg/Ca and Sr/Ca in calcite, aragonite, and seawater, respectively. At equilibrium, the partitioning constant between the two pure mineral phases depends on temperature, as the substitution of Mg into calcite and Sr into aragonite, are both associated with a change in enthalpy or heat of the reaction, which is sensitive to temperature. As the substitution of Mg into calcite is an endothermic reaction, the Mg/Ca ratio of calcite is expected to increase with increasing temperature (Rosenthal et al., 1997). In contrast, the

substitution of Sr into aragonite is an exothermic reaction, so the Sr/Ca ratio of aragonite should be inversely correlated with temperature. Based on thermodynamic considerations, Rosenthal et al. (1997) and Lea et al. (1999) proposed an exponential temperature dependence of Mg uptake into calcite of about 3% per K. This prediction is consistent with experiments of Mg incorporation into inorganically precipitated calcites; inorganic coprecipitation experiments of Sr with aragonite show a weak inverse linear dependence on temperature. However, as instructive as these data may be, biologically mediated processes are rarely at thermodynamic equilibrium. For example, Foraminifera contain 1–2 orders of magnitude lower Mg than found in marine inorganic calcites. Such offsets suggest that biological processes exert a major influence on the coprecipitation of metals in biogenic carbonates, thus highlighting the need for species-specific empirical calibrations.

Foraminiferal Mg/Ca Paleothermometry Temperature Calibrations of Mg/Ca in Planktonic Foraminifera The temperature dependence of Mg uptake into planktonic foraminiferal tests has been determined using three different approaches: (i) controlled culture experiments in which temperature is fixed independently of other environmental parameters (e.g., light, salinity, and pH); (ii) measurements of planktonic Foraminifera from sediment trap time series in sites characterized by significant seasonal variability in SST; (iii) analyses of fossil (dead) Foraminifera tests obtained from surface sediments, typically from core tops. A summary of published multiple-, and single-species calibrations is given in Table 1. At present, Mg/Ca calibrations are mostly expressed as an exponential dependence of temperature in the form: Mg/Ca (mmol mol1) ¼ B eAT where A and B are the exponential and pre-exponential constants, respectively, and T is temperature in  C. Justification for the choice of exponential fit to the empirical data comes primarily from the thermodynamic prediction of exponential response. In this relationship, the exponential constant reflects the Mg/Ca response to a given temperature change (in mmol mol1  C1), implying increased sensitivity with temperature. The pre-exponential constant determines the absolute temperature. Culture-based calibrations are available for several species including Globigerinoides sacculifer (Nu¨rnberg et al., 1996), Globigerinoides ruber (white; Kisakurek et al., 2008), Orbulina universa (Lea et al., 1999; Russell et al., 2004), Globigerina bulloides (Lea et al., 1999; Mashiotta et al., 1999), and Neogloboquadrina pachyderma (dextral; Langen et al., 2005) (Figure 1(a)). Combined, the available experiments suggest a temperature sensitivity of 9.4  0.9% (1 S.D.) change in Mg/Ca per  C, thus providing the strongest direct evidence for temperature control

871

872

PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES | Mg/Ca and Sr/Ca Paleothermometry

Table 1 Summary of published Mg/Ca–temperature calibrations for multiple and single species of planktonic Foraminifera (Mg/Ca in mmol mol1 and T in  C). Except when noted, all calibrations are based on exponential equations Mg/Ca ¼ A eBT Species

A

B

Material

Region

Multiple-species equations Two species Eight species Eight species

0.47 0.52 0.78

0.082 0.100 0.095

Core tops Core tops Core tops

Atlantic and Pacific North Atlantic North Atlantic

0.38 0.22

0.090 0.113

Sediment traps Core tops

North Atlantic North Atlantic

15–28 19–28

0.84

0.083

Core tops

North Atlantic

8–15

0.50 0.39 1.36 0.85 0.53 0.47 0.51 0.34 0.69 0.5 0.67 1.06 1.20 0.30 0.38 0.76 0.95 0.56 0.78 0.55 0.41 0.13

0.080 0.089 0.085 0.096 0.100 0.107 0.104 0.102 0.068 0.084 0.069 0.048 0.057 0.089 0.090 0.070 0.086 0.100 0.082 0.099 0.083 0.35

Culture Culture Culture Culture Culture Culture/core tops Culture Sediment traps Sediment traps Sediment traps Sediment traps Sediment traps Sediment traps Core tops Core tops Core tops Core tops Core tops Core tops Core tops Core tops Core tops

0.71 0.72 1.02 0.62 0.36 0.88

0.056 0.076 0.039 0.074 0.098 0.045

Core tops Core tops Core tops Core tops Core tops Core tops

Ten species Two shallow and thermocline dwellers Two deep dwellers Single-species equations G. ruber (white) G. sacculifer O. universa O. universa G. bulloides G. bulloides N. pachyderma (d) G. ruber (white; 250–350) G. ruber (white; 212–355) G. ruber (white; 250–350) G. sacculifer with sac (350–500) G. sacculifer w/out sac (350–500) G. bulloides (212–355) G. ruber (white; 250–350) G. ruber (white; 250–350) G. ruber (white; 250–315) O. universa G. bulloides G. bulloides (250–315) N. pachyderma (s) N. pachyderma (s) N. pachyderma (s) (linear equation Mg/Ca ¼ AT þ B) G. inflata (355–400) G. inflata (315–400) P. obliquiloculata (355–400) G. trancatuliniodes (r) (355–400) G. trancatuliniodes (r) G. trancatuliniodes (s) (355–400)

on planktonic foraminiferal Mg/Ca, much in the same way that inorganic experiments supported the thermodynamic expectations. Sediment trap calibrations provide additional support to the dominant temperature control. The sediment-trap based multispecies calibration of Anand et al. (2003) from the Sargasso Sea (Figure 1(b)) is in good agreement with culture-based calibrations, suggesting a multispecies temperature sensitivity of 9  0.3% per  C. Subsequent calibrations of G. ruber (white) from sediment traps in the Indian Ocean, off Java Island (Indonesia), and the South China Sea yield consistent results (Huang et al., 2008; Mohtadi et al., 2009). Consistent with these results, the multispecies core-top calibration of Elderfield and Ganssen (2000), reevaluated by Rosenthal and Lohmann (2002) suggests temperature sensitivity of 9.5 0.5% per  C (Figure 1(c)). A main advantage of multispecies calibration is the large temperature range it provides, which may offer a broader base

Temperature range ( C)

0–30 8–22 8–22

References

Nu¨rnberg et al. (1996) Elderfield and Ganssen (2000) Rosenthal and Lohmann (2002) based on data from Elderfield and Ganssen (2000) Anand et al. (2003) Regenberg et al. (2009) Regenberg et al. (2009)

North Atlantic Gulf of California Java, Indonesia North Atlantic North Atlantic Gulf of California Equatorial Pacific Atlantic and Pacific Atlantic North Atlantic North Atlantic Atlantic Norwegian Sea South Atlantic Norwegian Sea

17–32 19–30 16–25 15–25 16–25 10–25 9–19 22–28 20–33 25–30 22–28 22–28 16–31 24–29 21–29 17–28 8–22 8–22 10–18 1–12 0–15 0–15

Kisakurek et al. (2008) Nu¨rnberg et al. (1996) Lea et al. (1999) Russell et al. (2004) Lea et al. (1999) Mashiotta et al. (1999) Langen et al. (2005) Anand et al. (2003) McConnell and Thunell (2005) Mohtadi et al. (2009) Anand et al. (2003) Anand et al. (2003) McConnell and Thunell (2005) Lea et al. (2000) Dekens et al. (2002) Cleroux et al. (2008) Hathorne et al. (2003) Elderfield and Ganssen (2000) Cleroux et al. (2008) Nu¨rnberg (1995) Nu¨rnberg (1995) Kozdon et al. (2009)

Atlantic South Atlantic Atlantic Atlantic Indian Ocean Atlantic

11–18 3.1–16.5 20–25 11–17 8–25 12–17

Cleroux et al. (2008) Groeneveld and Chiessi (2011) Cleroux et al. (2008) Cleroux et al. (2008) McKenna and Prell (2005) Cleroux et al. (2008)

for statistical analysis. However, offsets among species can bias the calibration, and therefore, should be considered when choosing the appropriate equation, be it a single- or a multispecies calibration (Regenberg et al., 2009). Indeed, such offsets in Mg/Ca among individual species stress the need for single-species calibrations. Although there is significant variability among the different equations, the median exponential value of the 27 calibrations given in Table 1 is 8.5  1.6% (1 S.D.) per  C (average 8.2%), in accordance with the multispecies calibrations. Collectively, a sensitivity of 9  1% per  C seems to be a robust feature of planktonic Foraminifera; differences among calibration may reflect additional controls. It is also noteworthy that for single-species calibrations based on a narrow temperature range, a linear regression might provide a better fit to the data. The pre-exponential constant (B) differs significantly among calibrations, for both the same and

PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES | Mg/Ca and Sr/Ca Paleothermometry

G. bulloides (1) O. universa (1) O. universa (2) N. pachyderma (d) (3) G. ruber (4)

6

5

12

Mg/Ca (mmol mol-1)

Mg/Ca (mmol mol-1)

16

873

8

4

4

3

G. truncatulinoides G. sacculifer G. sacculifer with sac G. ruber (white) G. ruber (pink) P. obliquiloculata G. inflata G. hirsuta N. dutertrei G. carssiformis G. conglobatus

2 Mg/Ca = 0.38 exp(0.09T)

0 0 (a)

5

10

15

20

25

1 10

30

Temperature (⬚C)

15

20

25

30

Temperature (⬚C)

(b)

Mg/Ca (mmol mol-1)

4 G. bulloides G. ruber G. sacculifer G. siphonifera N. pachyderma G. hirsuta G. inflata G. truncatulinoides

3

2

1 (1) Mg/Ca = 0.52 exp(0.10T ) (2) Mg/Ca = 0.78 exp(0.095T)

0 (c)

0

5

10

15

20

Temperature (⬚C)

Figure 1 Mg/Ca–temperature calibrations based on: (a) culture experiments (data from (1) Lea et al. (1999), (2) Russell et al. (2004), (3) (Langen et al. (2005), (4) Kisakurek et al. (2008)); (b) sediment traps from the Sargasso Sea (Anand et al., 2003); (c) core tops (Elderfield and Ganssen, 2000) (regression line 1 from Elderfield and Ganssen (2000), line 2 from Rosenthal and Lohmann (2002) based on the same data set).

different species. This high variability reflects a combination of several factors including intra- and interspecies variability (Anand et al., 2003; Elderfield and Ganssen, 2000), diagenetic overprints including postdepositional dissolution (Rosenthal and Lohmann, 2002), contamination from sedimentary contaminants (e.g., Lea et al., 2005; Pena et al., 2005, 2008). It is also important to note that methodological biases among laboratories should be considered and accounted for when comparing data sets cleaned with and without the reductive step (Rosenthal et al., 2004). It follows, that the accuracy of Mg/Ca thermometry is often better in estimating relative changes in seawater temperatures (i.e., anomalies) than absolute temperatures. The good agreement between the core-top, sediment trap and culture-based calibrations also provides strong evidence that the temperature signal imprinted during the test formation is reliably transferred into the sediment although, as discussed below, secondary effects of salinity and pH on the calcification process and postdepositional alteration may complicate the interpretation of Mg/Ca records in terms of variability due strictly to temperature.

Inter- and Intraspecies Variability Interspecies variability in Mg/Ca is generally correlated with their calcification depth. Accordingly, shallow mix-layer dwellers (e.g., G. ruber and G. sacculifer) have high Mg/Ca, whereas deep dwellers (e.g., Pulleniatina obliquiloculata, Globigerinoides inflata, Globigerinoides truncatulinoides), are characterized by relatively low Mg/Ca (Rosenthal and Boyle, 1993). By assuming specific calcification depth to each foraminiferal species, and using the appropriate calibration (Anand et al., 2003; Cleroux et al., 2008; Regenberg et al., 2009; Skinner and Elderfield, 2005) one can reconstruct the temperature structure of the upper thermocline (e.g., Groeneveld and Chiessi, 2011; Xu et al., 2008). However, although it is common to assign planktonic Foraminifera specific calcification depths, it is also well known that many species continue to calcify while migrating vertically through the water column and some species, such as G. sacculifer, also add an outer calcite crust prior to reproduction (aka ‘gemetogenic crust’) at depths significantly deeper (and colder) than their principal habitat depth. These processes are likely to cause significant variability in the Mg

874

PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES | Mg/Ca and Sr/Ca Paleothermometry

distribution within and among Foraminifera tests, which should be linked to changes in calcification temperature. It follows, that the ‘whole test’ Mg/Ca composition most commonly used for temperature reconstructions represents a weighted average of calcite layers formed at different depths/ temperatures. This is less of an issue for the shallow, mixedlayer species of G. ruber and G. sacculifer and more critical for deep species such as P. obliquiloculata, Neogloboquadrina dutertrei, Globigerinoides tumida and G. trancatulinoides. There is also significant size-dependent variability in Mg/Ca ratios among tests of the same species. Mg/Ca increases with increasing test size in most species, possibly because small individuals calcify faster than large individuals (Elderfield et al., 2002) or because larger individuals spend relatively more time near the sea surface, where the water is warmer and the higher light intensity increases symbiont photosynthetic activity, which in turn allows growth to larger sizes (Honisch and Hemming, 2004; Spero and Lea, 1993). In addition, Mg/Ca can vary among different morphotypes of the same species, reflecting differences in growth season (e.g., the white vs. pink variety of G. ruber; Anand et al., 2003) or depth habitat (e.g., G. ruber s.s. vs. G. ruber s.l.; Steinke et al., 2005). The intraspecies variability implies that Mg/Ca analyses and calibration should be performed on the same morphotypes and within a narrow size range. Also, when possible, the same size fraction should be used for both Mg/Ca and isotopic analyses. Studies using microanalytical techniques also reveal large variability in Mg/Ca (and other trace elements) within and among individual tests that is apparently too large to be explained by the temperature change due to the vertical migration experienced by the different species during their adult life cycle (Hathorne et al., 2009; Kunioka et al., 2006; Sadekov et al., 2008). The pattern of Mg/Ca variation is significantly different between symbiont-bearing and symbiont-free species (Eggins et al., 2003, 2004). Tests of symbiont-bearing Foraminifera show cyclic variations between high and low Mg/Ca layers within individual chambers, which are largely absent from symbiont-free species (Sadekov et al., 2005). There are also indications that high Mg/Ca ratios are associated with organic-matrix rich bands (Kunioka et al., 2006). Nonetheless, despite the large, hitherto unexplained, intratest variability, it is widely accepted now that whole test Mg/Ca measurements produce estimates that closely match measured temperatures along the water column at the assumed individual species calcification depth and over a large geographical extent (Eggins et al., 2003; Hathorne et al., 2009; Sadekov et al., 2008, 2009, 2010).

Secondary Nontemperature Effects Salinity and pH effect Culture studies have provided evidence for a positive relationship between planktonic foraminiferal Mg/Ca and seawater salinity (Lea et al., 1999; Nu¨rnberg et al., 1996) and arguably an inverse dependence on pH (Lea et al., 1999; Russell et al., 2004). These experiments have documented a relatively small salinity effect, about 6  4% change in Mg/Ca per salinity unit (psu) over a salinity range of 20–45 for G. ruber, G. sacculifer and O. universa (Duen˜as-Boho´rquez et al., 2009; Kisakurek et al., 2008; Lea, et al., 1999). In contrast, core-top calibrations suggest substantially higher and quite variable salinity

dependence. For G. ruber, these calibrations suggest salinity sensitivity of about 14%, 27%, and up to 40% per psu. for sediments from the equatorial Indian-Pacific Oceans (MathienBlard and Bassinot, 2009), Atlantic Ocean (Arbuszewski et al., 2010), and Mediterranean Sea (Ferguson et al., 2008), respectively (although note that the very high estimates might reflect contamination by secondary high-Mg overgrowths; Hoogakker et al., 2009). Interestingly, the core-top calibrations suggest that the dependence on salinity is significantly higher than suggested from culture experiments and apparently increases with salinity. Indeed, a recent paper suggests little or no Mg/Ca dependence at low salinities (32–34) and a high dependence at high salinities (>35) (Arbuszewski et al., 2010). The pH effect on planktonic Foraminifera is still uncertain. In early culture experiments, Mg/Ca in O. universa and G. bulloides decreased by about 7  6% per 0.1 pH unit (Lea et al., 1999; Russell et al., 2004). Subsequent culture experiments of G. ruber and G. sacculifer suggest a negligible effect at modern seawater pH or [CO3] concentration and an inverse relationship below that (Duen˜as-Boho´rquez et al., 2009; Kisakurek et al., 2008). Indeed, the [CO3] effect may differ among species and also depend on other environmental conditions. For example, a sediment trap study in the Antarctic Ocean shows significant positive relationship between Mg/Ca of N. pachyderma (sinistral) and surface water [CO3] (Hendry et al., 2009). This and complementary studies in the Nordic Seas (Kozdon et al., 2009; Meland et al., 2006) suggest that temperature may exert the dominant control on Mg/Ca in N. pachyderma (s) above 3  C, whereas below that there is an additional saturation effect that might mask the temperature dependence. It could also be that long-term adaptation in the field induces a different response than observed under shortterm culture experiments. Whereas more studies are required to accurately determine the sensitivity of Mg/Ca to different environmental conditions, a new generation of equations may be required in order to more accurately describe the dependence of planktonic foraminiferal Mg/Ca on temperature, salinity, and carbonate concentrations. These multivariate equations should be used in future studies and for reappraising published records (e.g., Arbuszewski et al., 2010; Mathien-Blard and Bassinot, 2009).

Dissolution Effects Core-top studies along bathymetric transects indicate a systematic decrease in Mg/Ca ratios of planktonic Foraminifera with increasing depth, independent of the overlying SSTs, thereby suggesting that foraminiferal Mg/Ca is altered by postdepositional dissolution on the seafloor, driven by the depthdependent decrease in carbonate saturation levels (Dekens et al., 2002; Rosenthal et al., 2000). The dissolution effect is significantly stronger below the lysocline, although the exact response depends on the species (Regenberg et al., 2006). It has been argued that the systematic decrease in Mg/Ca ratios of planktonic Foraminifera is likely caused by preferential dissolution of the Mg-rich and more dissolution-susceptible calcite (Brown and Elderfield, 1996), which results in a decrease in the whole test Mg/Ca ratio that shifts the temperature estimates toward apparently colder values (Rosenthal et al., 2000). The decrease in Mg/Ca with depth is higher in

PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES | Mg/Ca and Sr/Ca Paleothermometry

nonspinose, thermocline dwelling species (e.g., G. tumida and N. dutertrei) than in spinose, shallow dwelling species (G. ruber and G. sacculifer), in accordance with the larger range of temperatures over which the deep dwellers calcify (Brown and Elderfield, 1996; Dekens et al., 2002; Huang et al., 2008). Various approaches have been offered to account for dissolution effects on Mg/Ca. Several studies propose that combining core-top measurements, where the magnitude of the depth-dependent decrease of Mg/Ca has been determined with independent estimates for past shifts in lysocline depth, could be used to estimate the bias in Mg/Ca–temperatures due to dissolution effects (Dekens et al., 2002; Lea et al., 2000; Regenberg et al., 2006). Applying this approach to two Pacific records, these studies suggest a potential error of 0.5  C in the estimates of the tropical Pacific glacial–interglacial temperature amplitude. This approach, however, does not quantify the dissolution effects on specific samples, which might respond more to changes in pore-water chemistry than to shifts in the hydrographic lysocline depth. Taking a different approach, Rosenthal and Lohmann (2002) suggested that the relationship between size-normalized test weight, and the dissolutiondriven decrease in Mg/Ca, could be used for correcting down core Mg-based temperature estimates for dissolution effects. While this method is very attractive, as it directly addresses

Table 2 Species

875

dissolution effects experienced by a particular sample, further studies have demonstrated that the test’s size–weight relationship is not strictly a function of dissolution, as initially thought, but also depends on the [CO3] content of seawater in which they grow (Barker and Elderfield, 2002). It is possible that using multiple species at different water depths (i.e., above and below the lysocline) may provide a way to distinguish primary from postdepositional effects.

Temperature calibrations of Mg/Ca in benthic Foraminifera Initial calibrations, largely based on the Cibicidoides pachyderma from a shallow bathymetric transect in Little Bahama Banks (spanning a temperature range of 5–18  C), suggested that the temperature sensitivity of Cibicidoides spp. Mg/Ca is essentially identical with the 9% increase in Mg/Ca per  C observed in planktonic Foraminifera (Lear et al., 2002; Martin et al., 2002; Rosenthal et al., 1997). Similar temperature sensitivities were observed in other benthic species including Planulina spp., Oridorslis umbunatus, and Melonis spp. (Lear et al., 2002), however, as is the case for planktonic Foraminifera, it was also noted that interspecies offsets in benthic Foraminifera exist for which corrections are needed (e.g., Lear et al., 2000). Subsequent studies have raised questions, however, about the accuracy of these initial calibrations (Table 2).

Summary of published Mg/Ca–temperature calibrations for benthic Foraminifera (Mg/Ca in mmol mol1 and temperature in  C) A

B

Exponential equations Mg/Ca ¼ A eBT C. pachyderma 1.36 0.100 C. pachyderma, C. 0.85 0.11 wuellerstorfi C. pachyderma, C. 0.87 0.11 wuellerstorfi, C. compressus Planulina spp. 0.79 0.120 O. umbonatus 1.01 0.114 O. umbonatus 1.53 0.09 Melonis spp. 0.98 0.101 Melonis barleeanus 0.658 0.137 P. ariminensis 0.91 0.062 Cassidulina neoreretis 0.864 0.082 Gyroidinoides spp. 1.71 0.109 Uvigerina spp. 0.92 0.061 Uvigerina spp. 0.98 0.043 Uvigerina spp. 0.98 0.045 Linear equations Mg/Ca ¼ A þ BT C. pachyderma 1.12 0.12 C. pachyderma 0.89 0.13 Uvigerina spp. 0.77 0.079 Uvigerina spp. 0.98 0.065 Planoglabratella 89.7 2.22 opercularis Quinqueloculina yabei 66.0 2.9 Planoglabratella 81.5 1.6 opercularis Hyalinea balthica 0 0.488 Hoeglundona elegans 0.96 0.034 Linear equations Mg/Ca ¼ A þ BT þ CD[CO3] C. wuellerstorfi 0.82 0.056

C

0.0087

Mineral

Region

Temperature range ( C)

References

Low-Mg calcite Low-Mg calcite

Little Bahama Banks Atlantic and Pacific

4.5–18 0–18

Rosenthal et al. (1997) Martin et al. (2002)

Low-Mg calcite

Atlantic and Pacific

1–18

Lear et al. (2002)

Low-Mg calcite Low-Mg calcite Low-Mg calcite Low-Mg calcite Low-Mg calcite Low-Mg calcite Low-Mg calcite Low-Mg calcite Low-Mg calcite Low-Mg calcite Low-Mg calcite

Pacific Atlantic and Pacific Atlantic Atlantic and Pacific Iceland Pacific Iceland Mediterranean Sea Atlantic and Pacific Florida Straits South Pacific

2–12 1–10 3–10 1–18 0–7 3–15 8–14 0–7 2–18 5–18 1–20

Lear et al. (2002) Lear et al. (2002) Rathmann et al. (2004) Lear et al. (2002) Kristja´nsdo´ttir et al. (2007) Lear et al. (2002) Kristja´nsdo´ttir et al. (2007) Cacho et al. (2006) Lear et al. (2002) Marchitto et al. (2007) Elderfield et al. (2009)

Low-Mg calcite Low-Mg calcite Low-Mg calcite Low-Mg calcite High-Mg calcite

Florida Straits Little Bahama Banks Florida Straits south Pacific culture

5–18 5–18 5–18 1–20 10–25

Marchitto et al. (2007) Curry and Marchitto (2008) Marchitto et al. (2007) Elderfield et al. (2009) Toyofuku et al. (2000)

High-Mg calcite High-Mg calcite

culture culture

10–25 10–25

Toyofuku et al. (2000) Toyofuku and Kitazato (2005)

Low-Mg calcite Aragonite

Global Global

5–14 >4

Rosenthal et al. (2011) Rosenthal et al. (2006)

Low-Mg calcite

Global

<3

Elderfield et al. (2006)

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PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES | Mg/Ca and Sr/Ca Paleothermometry

First, it has been argued that the shallow Little Bahama Banks samples are compromised by diagenetic calcite overgrowths enriched in Mg, thus yielding an apparently high exponential dependence on temperature. New calibration studies of C. pachyderma from depth transects in the Florida Straits and Great Bahama Banks, in areas where the problem of diagenetic calcite overgrowths is apparently minimal, suggest a linear relationship between Mg/Ca and BWT with a sensitivity of 0.12 mmol mol1  C1 (Table 2) for the temperature range from 5.8 to 18.6  C (Marchitto et al., 2007). This calibration has been replicated in Little Bahama Banks by Curry and Marchitto (2008) who used a microanalytical technique to avoid contamination by diagenetic overgrowths. Secondly, in deep waters the degree of CaCO3 saturation starts to exert a significant control on the incorporation of Mg into benthic foraminiferal tests, thus altering the Mg/Ca– temperature relationships (Elderfield et al., 2006; Martin et al., 2002; Rosenthal et al., 2006). New calibrations based on global datasets suggest a D[CO3]-Mg/Ca sensitivity of 0.009 mmol mol1 mmol1 kg1 (where D[CO3] is the degree of calcite bottom-water saturation) in calcitic benthic tests that, in the modern ocean, becomes increasingly important below 3  C (Elderfield et al., 2006; Healey et al., 2008). There are also indications for a significant [CO3] ion effect at warmer, oversaturated waters (Bryan and Marchitto, 2008). After adjusting for the additional D[CO3] effect, a sensitivity of about 0.12–0.15 mmol mol1  C1 is found in other epifaunal species including Cibicidoides wuellerstorfi, Cibicidoides mundulus and Oridorsalis umbonatus (Healey et al., 2008; Raitzsch et al., 2008; Sosdian and Rosenthal, 2009; Yu and Elderfield, 2008), consistent with the C. pachyderma calibration (Marchitto et al., 2007). It is now evident that benthic foraminiferal Mg/Ca thermometry is not as straightforward as initially thought, and the additional D[CO3] effect needs to be constrained in any down core reconstructions of deep-water temperatures. One possible approach is using paired elemental ratios with different sensitivities for temperature and D[CO3] (e.g., Mg/Ca coupled with Li/Ca or B/Ca) to correct for, or deconvolve the contribution of different environmental parameters (Bryan and Marchitto, 2008; Lear et al., 2010; Yu and Elderfield, 2008) or through using independent estimates of the carbonate ion effect (Sosdian and Rosenthal, 2009). However, the low sensitivity suggested for the above-mentioned Foraminifera and the additional D[CO3] effect can also lead to large errors associated with estimating deep ocean temperatures (e.g., 2.4  C for C. pachyderma: Marchitto et al., 2007). Alternatively, it has been argued that infaunal taxa such as Uvigerina sp. may be less affected by the D[CO3] effect as it calcifies from saturated pore waters and thus is less affected by bottom water chemistry (Elderfield et al., 2009). The exact temperature sensitivity of Uvigerina species is, however, still uncertain. On the other hand, core-top calibrations suggest sensitivity of about 6–8% per  C (Elderfield et al., 2009; Lear et al., 2002), so a significantly higher sensitivity is required to explain the relatively large down core variability (Elderfield et al., 2009; Martin et al., 2002). If indeed some shallow infaunal benthic Foraminifera are free of saturation effects and characterized by high temperature sensitivity, they may offer a new tool for accurate and

precise reconstruction of past deep ocean temperatures. One promising example is the use of the shallow infaunal calcitic species, Hyalinea balthica, which exhibits high temperature sensitivity (0.5 mmol mol1  C1) and no discernible D[CO3] effect (Bamberg et al., 2010; Rosenthal et al., 2011).

Inferring Seawater d18O and Salinity The d18O composition of calcareous marine fossils reflects, at a minimum, both the temperature and the isotopic composition of seawater in which the shell or skeleton was formed. The d18O of modern surface water reflects, in part, the hydrological balance between evaporation and precipitation (E/P), that is, the surface salinity. Advection can also alter d18Osw by mixing waters of different salinities (or d18Osw). Over very long time scales, seawater d18O is also a function of the extent of continental ice sheets. Paired d18O and Mg/Ca measurements in Foraminifera allow for the adjustment of temperaturedependency of carbonate d18O and isolation of d18Osw (Lea et al., 2000; Mashiotta et al., 1999), which can then be used to reconstruct local changes in E/P and by inference salinity (e.g., Oppo et al., 2009; Schmidt et al., 2004; Stott et al., 2004), and provide valuable information about changes in continental ice volume (e.g., Lear et al., 2000). A major advantage of measuring both proxies on the same samples is that it allows for the determination of temporal leads and lags between variations in seawater temperature and the build-up or erosion of continental ice sheets, which is fundamental for understanding climate change. In the past decade, the application of this method has already led to important findings. New Mg-derived SST records have contributed to our understanding of the role of the equatorial Pacific in global climate change on millennial (e.g., Koutavas et al., 2006; Linsley et al., 2010; Stott et al., 2002), orbital (e.g., Lea et al., 2000; Rosenthal et al., 2003), and secular time scales (e.g., de Garidel-Thoron et al., 2005; Wara et al., 2005). Among the most significant findings that come from these studies are: (1) the equatorial Pacific was characterized by a ‘super El Nino’ state during the mid-Pliocene warm period (Wara et al., 2005); (2) except for glacial–interglacial variability, Western Pacific warm pool (WPWP) SSTs were stable throughout the Pleistocene, despite Northern Hemisphere glaciations and global cooling (de Garidel-Thoron et al., 2005); (3) SST in the WPWP was 2–3  C cooler during glacial periods than at present (e.g., de Garidel-Thoron et al., 2007; Lea et al., 2000; Linsley et al., 2010); and (4) during glacial terminations, changes in equatorial SST arguably led the Northern Hemisphere deglacial record by 3000 years (Lea et al., 2000; Visser et al., 2003). Other examples of a breakthrough offered by this new approach are benthic foraminiferal Mg/Ca records documenting the global cooling trend throughout the Cenozoic (Billups, 2003; Billups and Schrag, 2002; Lear et al., 2000, 2004; Sosdian and Rosenthal, 2009). The low-resolution seawater oxygen isotope record (d18Oseawater) of Lear et al. (2000) suggests that the Cenozoic expansion of continental ice sheets occurred primarily in three steps: the Antarctic glaciation during the Eocene–Oligocene (E/O) transition and the middle Miocene, and the Northern Hemisphere glaciation during the PlioPleistocene. A high-resolution benthic Mg/Ca record from the

PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES | Mg/Ca and Sr/Ca Paleothermometry North Atlantic (Figure 2) shows two distinct cooling events: the first associated with the late Pliocene (LPT, 2.5–3 Ma) and the second preceding the transition from a dominant 41–100 ka climate variability during mid-Pleistocene (the Mid-Pleistocene Transition, 1.2–0.85 Ma; Sosdian and Rosenthal, 2009). The reconstructed d18Oseawater record suggests that sea level was apparently 25  20 m higher than at present during the mid-Pliocene, a period considered to be a good analog for future climate. While this estimate needs to be validated, given the uncertainties in the method, it offers a new way to assess climate variability and impacts under different boundary conditions. On time scales longer than 1 My, secular variations in seawater Mg/Ca need to be considered before interpreting foraminiferal Mg/Ca records strictly in terms of temperature. First, the paleotemperature equation should include a term accounting for the difference between the seawater Mg/Ca ratio in the past and in the modern ocean. Secondly, studies of various calcareous organisms suggest that the partitioning of Mg between the solution and biogenic CaCO3 minerals is nonlinearly dependent on the Mg/Ca ratio of the seawater (Hasiuk and Lohmann, 2010; Ries, 2004; Segev and Erez, 2006). Therefore, these parameters need to be considered in order to obtain accurate reconstructions of the secular variations of ocean temperature (e.g., Cramer et al., 2011 and references therein).

877

Marine Ostracode Mg/Ca Paleothermometry Studies over the past decade have provided strong evidence for the temperature dependence of Mg/Ca in marine ostracodes (Dwyer et al., 2002, and references therein). Indeed records of Mg/Ca in marine ostracodes have been used successfully for reconstructing the history of BWTs through the Pliocene and Pleistocene (e.g., Cronin et al., 2005; Dwyer et al., 1995). Ostracodes are bivalved crustaceans that grow via a process of molting. Ostracodes excrete an exoskeleton known as a carapace that in some taxa consists of 80–90% calcite and the rest of chitin and proteins. Tests of adult specimens, used in paleoceanography, are flat, typically 0.5–1 mm in length and weigh between 20 and 200 mg, thus providing a good target for geochemical analyses. Temperature calibrations are currently available for two marine, epifaunal benthic ostracodes, the deep-sea genus Krithe and shallow marine/estuarine genus Loxoconcha (Dwyer et al., 1995, 2002), both long-lived genera known since the Cretaceous. Mg/Ca ratios in these genera are significantly higher than in deep-sea benthic Foraminifera. Based on core-top calibrations, Dwyer et al. (2002) proposed a linear relationship between Krithe Mg/Ca and temperature, with a slope of 0.95  0.15 mmol mol1  C1 or expressed as an exponential dependence of 6% per  C, which is within the range of sensitivities observed in Foraminifera (Figure 3).

2.5

MPT

d 18Ob (‰)

3

LPT

3.5 4 4.5 5.5 2.2 2 1.8 1.6 1.4 1.2

(a) 0

500

1000

1500

2000

2500

3000

8 6 4 2 0

1 0.8

Bottom water temperature (⬚C)

Mg/Ca (mmol mol-1)

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-2 40 0

0

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0.4 0.8

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Figure 2 Plio-Pleistocene benthic foraminiferal records from Deep Sea Drilling Project (DSDP) site 607 (41 N, 32 W, 3427-m water depth) in the North Atlantic: (a) benthic oxygen isotope record (d18Ob in %PDB); (b) benthic foraminiferal Mg/Ca record, derived from C. wuellerstorfi and O. umbonatus, converted to bottom water temperature (BWT) using the equation: Mg/Ca ¼ 0.15 BWT þ 1.16; (c) seawater (d18Osw in %SMOW) and sea level estimates (green line represents three-point smoothed curve) derived from the Mg/Ca-BWT and d18Ob records. MPT and LPT refer to mid-Pleistocene and late Pliocene transitions. Black lines represent average signals for each variable determined by applying a Gaussian filter with a cut-off frequency of 400 ka. Modified from Sosdian S and Rosenthal Y (2009) Deep sea temperature and ice volume changes across the Pliocene–Pleistocene climate transitions. Science, 325: 306–310.

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PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES | Mg/Ca and Sr/Ca Paleothermometry

28

1 2 3 4

Mg/Ca (mmol mol-1)

24

Atlantic/Deep Pacific Chilean fiords

20

Arctic (0–900 m)

16

Arctic (900–4400 m) (1) Mg/Ca = 1.10T+7.10

12

(2) Mg/Ca = 1.05T+7.16 (3) Mg/Ca = 0.93T+8.24

8

(4) Mg/Ca = 0.724T+9.97

4 0 -2

0

2

4

6

8

10 12 14 16 18

Temperature (⬚C) Figure 3 Various Mg/Ca–temperature calibrations for adult shells of genus Krithe. Regression lines are based on samples from (1) Atlantic and central Pacific core tops; (2) data from regression 1 and core tops from Chilean Fjords; (3) same as 2 with Arctic core tops between 0 and 900 m; (4) same as 3 with Arctic core tops deeper than 900 m. Modified from Dwyer GS, Cronin TM, and Baker PA (2002) Trace elements in marine ostracodes. The Ostracoda: Applications in Quaternary Research, pp. 205–224. American Geophysical Union.

Significant differences between the two ostracode genera and ontogenic effects within each genus can be minimized by only studying tests of the adult stage for a particular genus (Dwyer et al., 2002). The offsets among the different calibrations suggest that secondary, nontemperature parameters might influence the Mg/Ca record of marine ostracodes. Culture studies show no discernible salinity effects, and dissolution experiments suggest that the effects of postdepositional dissolution on the test composition are minimal (Dwyer et al., 2002). A D[CO3] effect on Mg/Ca in ostracodes, which has not yet been fully investigated, might explain the divergence among the calibrations.

Coralline Sr/Ca Paleothermometry Temperature Calibrations of Sr/Ca in Corals The variability of skeletal Sr/Ca in hermatypic coral skeletons has been shown in many studies to be highly correlated with monthly or seasonal variations in water temperature. Early studies indicated that the Sr/Ca in coral skeletal aragonite is primarily regulated by water temperature (Beck et al., 1992) with more Sr incorporated in coral skeleton at lower temperatures, as discussed above. The relationship obtained in these studies is of the form:
ðSr=CaÞcoral mmol mol1 ¼ B þ AðSSTÞ  C, and the slope where SST is given in A ¼ 0.062 mmol mol1  C1 (Beck et al., 1992). These initial observations are supported by more recent studies showing strong correlations between near-monthly coral Sr/Ca and surface water temperature (e.g., Alibert and McCulloch, 1997; DeLong et al., 2007; Goodkin et al., 2007; Linsley et al., 2000, 2004; Marshall and McCulloch, 2002; Nurhati et al., 2009; Wu, 2010) (Figure 4). Generally, the slope of the Sr/ Ca–SST relationship is similar for near-monthly coral Sr/Ca and monthly SST (Table 3). Despite the success of the coral Sr/ Ca paleo-thermometer at many sites, these and other studies have also revealed potential complicating factors that can limit

the use of this tracer. Some of these factors–uncertainties are summarized below.

Secondary Nontemperature Effects Disequilibrium offset In Porites corals, although the slope of the Sr/Ca–temperature relationships derived at different sites with different individual corals is generally the same, the intercepts of the linear relationship can be significantly different among sites, even for coral analyzed on the same type of instrument. Marshall and Marshall and McCulloch (2002), working with Porites corals from the Great Barrier Reef, attribute this effect to be due in part to the disequilibrium Sr/Ca offset in corals. Similar to the 18 O/16O in corals, the skeletal Sr/Ca is out of equilibrium with seawater Sr/Ca. Using three intracolony and three intercolony Porites coral records from the reefs offshore of Ame´de´e Island, New Caledonia, DeLong et al. (2007) found the coral Sr/Ca signal over the twentieth century to be highly reproducible. They report that the average absolute offset between coeval monthly Sr/Ca determinations between any two coral time series is 0.035  0.026 mmol mol1 (0.65  C), which is less than twice the analytical precision of the coral Sr/Ca measurements. At Clipperton Atoll (10 N, 109 W) in the eastern Pacific, Wu (2010) and (Wu et al. (in review) observed an offset in mean Sr/Ca value (mmol mol1) between three Porites lobata colonies as previously observed in other coral skeletal Sr/Ca records (Bagnato et al., 2004; De Villiers et al., 1995; DeLong et al., 2007; Marshall and McCulloch, 2002). The offsets between the cores ranged from a low of 0.04 mmol mol1 to a high of 0.09 mmol mol1, which would equate to a 0.5–1.3  C difference in temperature reconstruction. In Savusavu Bay on Fiji’s north island of Vanua Levu (16 490 S, 179 100 E), Wu (2010) observed mean Sr/Ca offset of 0.37 mmol mol1 between the two adjacent Fiji Porites colonies in the same water depth. The colonies are thought to be the same species, based on morphology and skeletal architecture. It is worth noting that the two coral cores were analyzed

PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES | Mg/Ca and Sr/Ca Paleothermometry

879

SST C (22 S, 160 W) 29 28 27 26 25 24 23 22

X-ray of Porites lobata

1997

April 1997

1995

1993

1990

Year

1991

1989

1987

1985

1980 1983

1981 9.2 9.3 9.4 9.5 9.6 9.7 9.8 Sr/Ca (mmol mol-1) Figure 4 Comparison of monthly sea surface temperature (red) for the latitude–longitude grid including Rarotonga in the South Pacific (1 by 1 ; centered at 22 S, 160 W) and Sr/Ca measurements (black) analyzed at 1 mm intervals in a Porites lutea coral core from Rarotonga. The x-ray positive of the coral core shows the sampling track and annual growth density bands.

Table 3

Several published monthly Sr/Ca–temperature calibrations for Porites corals from the Pacific Ocean (Sr/Ca in mmol mol1 and T in  C)

Site

New Caledonia New Caledonia Hawaii Palmyra Fanning Christmas (Pacific) Taiwan Great Barrier Reef Great Barrier Reef Fiji Rarotonga Tonga Clipperton Fiji Bermuda Bermuda Bermuda

Species

Porites lobata Porites lobata Porites lobata Porites sp. Porites sp. Porites sp. Porites lobata Porites sp. Porites sp. Porites lutea, Porites lutea Porites lutea Porites lutea Porites lobata Diploastrea (Septal) Diploria labyrinthiformis Diploria labyrinthiformis Diploria labyrinthiformis

Sr/Ca ¼ B þ AT B

A

10.479 10.451 10.956 11.43 10.78 11.18 10.286 10.38 10.4 10.63, 9.78 11.12 10.927 11.104 10.08 10.4 11.2 10.3

0.062 0.054 0.079 0.088 0.065 0.079 0.051 0.055 0.059 0.052, 0.034 0.065 0.071 0.068 0.034 0.048 0.084a 0.045

References

Method

Beck et al. (1992) DeLong et al. (2007) De Villiers et al. (1994) Nurhati et al. (2009) Nurhati et al. (2009) Nurhati et al. (2009) Shen et al. (1996) Alibert and McCulloch (1997) Marshall and McCulloch (2002) Linsley et al. (2004) and Wu (2010, PhD diss.) Linsley et al. (2000) Wu (2010, PhD diss.) Wu (2010, PhD diss.) Bagnato et al. (2004) Goodkin et al. (2007) Goodkin et al. (2007) Goodkin et al. (2007)

TIMS ICP-OES TIMS ICP-OES ICP-OES ICP-OES TIMS TIMS TIMS ICP-OES ICP-OES ICP-OES ICP-OES ICP-OES ICP-OES ICP-OES ICP-OES

TIMS, thermal ion mass spectrometry; ICP-OES, inductively coupled plasma optical emission spectrometer. a Before growth rate correction.

on exactly the same ICP-OES instrument in two different laboratories, following interlaboratory calibration (Harvard and Lamont Doherty). This large offset would equate to an unrealistic temperature reconstruction difference of 10  C based on the Sr/Ca–SST calibration derived in this study, or 6  C

based on the commonly accepted Sr/Ca–SST calibration of 0.06 mmol mol1  C1. The different intercepts for coral Sr/Ca–SST relationships shown in Table 3 reflect the fact that the disequilibrium offset can vary between corals of the same species, even in close

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PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES | Mg/Ca and Sr/Ca Paleothermometry

proximity. For the genus Porites, the similarity of the slopes of the Sr/Ca–SST relationships (Table 3) indicates that Sr/Ca can be used to accurately reconstruct paleo-water temperature, as long as adequate site- and coral-specific temperature calibrations are carried out. For fossil corals lacking a site-specific calibration, the consistent Sr/Ca–SST slope, but potentially different intercept implies that only relative changes in SST over time can accurately be reconstructed (0.5  C). In all cases, however, for an individual coral Sr/Ca record to be an absolute temperature proxy, the degree of disequilibrium offset must be constant in that coral. This is generally the case for corals sampled along the maximum growth axis (e.g., DeLong et al., 2007; Wu, 2010). Many other studies report no or insignificant Sr/Ca offsets from corals at the same or closely adjacent sites. For example, in the Line Islands, Nurhati et al. (2009) found insignificant Sr/Ca offsets between Porites corals from Christmas, Fanning, and Palmyra Atolls when measured in the same laboratory. They report correlation coefficients (r-values) between SST and monthly Sr/Ca of 0.80, 0.79, and 0.71 for Christmas, Fanning, and Palmyra, respectively.

Extension rate Some studies of slow growing corals have reported growth rate effects on skeletal Sr/Ca. De Villiers et al. (1994) reported significantly higher Sr/Ca ratios associated with slower skeletal extension rates in Pavona clavus. De Villiers et al. (1994) argued that the difference between skeleton accretion at 12 and 6 mm year1 is equivalent to 1–2  C based on near-monthly calibration with monthly SST. However, Marshall and McCulloch (2002) noted that because De Villiers et al. (1994) did not sample exclusively along the coral’s maximum growth axis, this invalidates their calcification rate argument, since it is widely known that sampling in off-axis sections of hermatypic coral skeletons generally yields higher Sr/Ca ratios, apparently due to smaller polyp size and lower skeletal density. In other slow growing corals, Bagnato et al. (2004) and Goodkin et al. (2007) both found strong correlations between Sr/Ca and SST in the slow growing corals Diploastrea and Diploria, respectively. Goodkin et al. (2007) also observed a skeletal extension rate correlation and developed a correction method for adjusted time series Sr/Ca for extension rate effects. In faster growing Porites corals, Shen et al. (1996) and Alibert and McCulloch (1997) find little growth rate effect on Sr/Ca in Porites lutea skeletons accreted in the maximum growth axis and Shen et al. (1996) found no perceptible growth rate effect in Porites at accretion rates between 18 and 23 mm year1. DeLong et al. (2007) analyzed Sr/Ca along several Porites cores and slabs from New Caledonia Porites and found excellent replication and no apparent skeletal extension rate effects. Wu (2010) measured 5-year average extension and calcification rates on Porites cores from Fiji, Tonga, and Clipperton. The results revealed no clear and consistent growth trends and no obvious growth effects on the geochemical proxies d18O, d13C, and Sr/Ca.

Monthly Sr/Ca oscillations in coral aragonite Using high spatial resolution ion microprobe analyses, Meibom et al. (2003) have found distinct near-monthly oscillations of Porites lutea skeletal Sr/Ca with an amplitude greater

than 10%. They propose that these ‘monthly’ Sr/Ca oscillations result from metabolic changes that are synchronous with the lunar cycle. As has been previously observed in Porites (Cohen et al., 2001), their study show that temporally equivalent skeletal elements in Porites can have different Sr/Ca ratios. Some of the skeletal Sr/Ca heterogeneity is apparently due to the presence of night-time and day-time deposited skeletal elements (Cohen et al., 2001). During the night, small micron-sized crystals are deposited at the axial spines of the corallites (centers of calcification). Larger needle shaped crystals are formed during the day along all surfaces of the precipitating skeleton. Microprobe analyses of Sr/Ca show that the night-time and day-time skeletons have significantly different Sr/Ca ratios (e.g., Cohen et al., 2001) and that Sr incorporation into the skeleton may be inversely related to calcification rate (FerrierPages et al., 2003).

Replication of decadal and long-term secular variability in coral Sr/Ca One means of potentially separating climate-related Sr/Ca variability from variability induced by biologic or diagenetic processes is to develop replicated coral Sr/Ca series from a specific site or in a specific region. Only limited replication of centuryscale and decadal Sr/Ca variability has been completed to date, and more replicated multicentury Sr/Ca corals records are needed to assess potential biologic or diagenetic effects on Sr/Ca. From the South Pacific, Linsley et al. (2006) present partially replicated annually averaged multicentury long Porites Sr/Ca series from Fiji and Rarotonga. In this study, Linsley et al. (2006) reported that application of the slope of Sr/Ca– SST relationship derived from near-monthly coral Sr/Ca and monthly SST over a calibration interval from 1970 to the core tops to the entire annually averaged multicentury Sr/Ca series at both sites results in unrealistically large amplitude long-term secular trends in SST. To get the Sr/Ca secular trend and interdecadal amplitude at Fiji and Rarotonga to fit the amplitude of instrumental SST and night time marine air temperature variability from 1870 to 1997 required a scaling of 0.2 mmol mol1  C1 whereas the monthly calibrations are 0.053 and 0.065 mmol mol1  C1 at Fiji and Rarotonga, respectively. This 0.2 mmol mol1  C1 scaling also resulted in a more realistic amplitude of estimated SST changes before 1870. At Fiji, Sr/Ca results for the 20-year period centered on 1660 would indicate a 4–5  C cooling relative to the late twentieth century using the calibration derived from comparison to monthly instrumental SST (Linsley et al., 2004). The apparent amplification of inferred SST changes from coral Sr/Ca prior to the calibration period was previously discussed with respect to interdecadal variability in Sr/Ca from both Fiji and Rarotonga by Linsley et al. (2004) and may suggest that at these sites the Sr/Ca–SST relationship has not been constant over the last 380 years. Replicated monthly resolution Sr/Ca time series from three P. lobata coral colonies at Clipperton Atoll (10 180 N, 109 130 W) in the eastern Pacific are used to assess the fidelity of coral Sr/Ca as a SST proxy in this region of relatively stable SST (Wu, 2010; Wu et al., in review). Significant correlations were found between individual cores and gridded instrumental SST for the calibration period (1981–94) with the three-core average increasing the correlation to SST. Down-core, the

PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES | Mg/Ca and Sr/Ca Paleothermometry

intercolony monthly Sr/Ca records showed good coherence with the exception of a 12-year section of growth in one colony. Although, Wu (2010) could find no evidence of diagenesis or other significant skeletal alterations in this section, the presence of this ‘anomalous’ interval serves to support the multicoral replication approach. Since AD 1900, there is an observed secular warming trend of 1  C recorded in the coral Sr/Ca records which is 0.5–0.75  C greater than the trend in instrumental SST. The decadal Sr/Ca variability is also greater than that observed in instrumental SST. The magnitude of these coral Sr/Ca–SST trends are in line with other coral Sr/ Ca reconstructions and further supports the conclusion that decadal and secular trends in SST at shallow reef sites are not fully reflected in gridded SST products. A similar amplification also appears in the results of a digenetically altered coral from Ningaloo reef in Western Australia reported by Mu¨ller et al. (2001). However, in New Caledonia Porites, it is noteworthy that a network of coral Sr/Ca records shows no apparent amplification of the long-term secular SST signal (DeLong et al., 2007).

Coral Sr/Ca Summary Coral skeletal Sr/Ca seasonal variability has proven to be highly correlated with monthly instrumental water temperature at many sites and appears to be a potentially useful tracer in paleoceanographic studies. As noted above, the results of several studies have raised questions/concerns about the accuracy of the coral Sr/Ca thermometer in some modern corals that are important since these issues may directly affect interpretation of Sr/Ca data from fossil corals and reports of significant tropical Pacific and Caribbean cooling prior to 10 000 years BP (e.g., Beck et al., 1997; Guilderson et al., 1994). Although this tracer clearly has the potential to greatly improve our understanding of oceanic conditions, there remains important work to be done to more fully understand the significance of all modes of Sr/Ca variability in corals.

See also: Paleoceanography: Paleoceanography An Overview. Paleoceanography, Biological Proxies: Benthic Foraminifera; Corals, Sclerosponges and Mollusks; Planktic Foraminifera. Paleoceanography, Physical and Chemical Proxies: Nutrient Proxies.

References Alibert C and McCulloch MT (1997) Strontium/calcium ratios in modern Porites corals from the Great Barrier Reef as a proxy for sea surface temperature: Calibration of the thermometer and monitoring of ENSO. Paleoceanography 12: 345–363. Anand P, Elderfield H, and Conte MH (2003) Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time-series. Paleoceanography 18(2): 1050. http://dx.doi.org/10.1029/2002PA000846. Arbuszewski J, deMenocal P, Kaplan A, and Farmer CE (2010) On the fidelity of shellderived d18Oseawater estimates. Earth and Planetary Science Letters 300: 185–196. Bagnato S, Linsley BK, Howe SS, Wellington GM, and Salinger J (2004) Evaluating the use of the massive coral Diploastrea heliopora for paleoclimate reconstruction. Paleoceanography 19(PA 1032): 1–12.

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