What is the ideal proxy of Palaeozoic seawater chemistry?

UWE BRAND, ALAN LOGAN, MARIA ALEKSANDRA BITNER, ERIKA GRIESSHABER, KAREM AZMY & DIETER BUHL

PROXIES of original seawater chemistry are abundant for the Palaeozoic with a choice of brachiopods, conodonts or whole rocks available to the investigator (e.g., Brand 2004). Articulated brachiopod shells consist of biogenic low-Mg calcite (bLMC) that is relatively resistant to diagenesis (e.g., Al-Aasm & Veizer 1982), conodonts consist of francolite (cF) a carbonate fluorapatite, which is either superior in its resistance to diagenesis compared to that of bLMC (e.g., Joachimski et al. 2009) or represents a diagenetic replacement phase of the original biogenic apatite (e.g., Iacumin et al. 1996), and whole rock consists of a mélange of carbonate minerals that has varied with geological time but is always altered to diagenetic low-Mg calcite (dLMC; e.g., Brand & Veizer 1980).

The use of any one particular proxy is generally influenced by many considerations;

among the foremost is their ease of availability in the rock sequence, which is supported by the overwhelming use of whole rock as a seawater proxy (e.g., Denison et al. 1994; Buggisch et al. 2003). However, the use of whole rock as a proxy needs a caveat of caution, because the mineralogy of the ocean may have varied with geological time between aragonite- and calcite-dominated seas (e.g., Sandberg 1983; Fig. 1) and thus a direct comparison and evaluation of chemistry may encounter some difficulties. All of the aforementioned proxies have been used to infer the chemistry of the ambient seawater and used the results in modelling palaeohydrospheric, atmospheric and climatic changes of the Palaeozoic (e.g., Popp et al. 1986b; Brand 1989; Bertram et al. 1992; Mii et al. 1999; Veizer et al. 1999; Joachimski & Buggisch 2002; Buggisch et al. 2003; van Geldern et al. 2006).

BRAND, U., LOGAN, A., BITNER, M.A., GRIESSHABER, E., AZMY, K. & BUHL, D., 2011:??:??. What is the ideal proxy for Palaeozoic seawater chemistry? Memoirs of the Association of Australasian Palaeontologists 41, 9-24. ISSN 0810-8889.

The chemistry of many biogenic allochems and whole rock is used as a proxy of original seawater chemistry during the geological past. Common Palaeozoic proxies are brachiopods, conodonts, and whole rocks. Brachiopods may secrete low-Mg calcite shells, conodonts consist of francolite, a carbonate fluorapatite, and whole rock is usually diagenetic low-Mg calcite altered from different original carbonate mineralogy. Stratigraphic uncertainty of specimens from isolated successions remains a serious impediment to accurate age assignment and in modelling events of the ancient hydrosphere. Furthermore, diagenesis is the great nemesis of these allochems and whole rocks, and in many instances they are not preserved in their original composition. Thus, the chemical composition of many of these proxies instead of being representative of the original and ambient seawater reflects the composition of the diagenetic fluid and character of the diagenetic microenvironment such as the water/rock ratio. Consequently, the best proxy is not necessarily the most abundant material, nor the most readily available (i.e. in outcrop or museums), nor the one supported exclusively by hypothetical concepts. Instead, the best proxy is the one that has passed the most screening tests, and in addition, is stratigraphically well constrained and provides results reflecting the natural variation of the ambient oceanographic environment.

Uwe Brand (uwe.brand@brocku.ca), Department of Earth Sciences, Brock University, St. Catharines, ON L2S3A1 Canada; Alan Logan, Centre for Coastal Studies, University of New Brunswick, Saint John, NB E2L 4L5 Canada; Maria Aleksandra Bitner, Institute of Paleobiology, Polish Academy of Sciences, ul. Twarda 51/55, 00-818 Warszawa, Poland; Erika Griesshaber, Department of Earth and Environmental Sciences, Ludwig-Maximilian University, Munich, Germany; Karem Azmy, Department of Earth Sciences, Memorial University, St. John’s, NL A1B 3X5 Canada; Dieter Buhl, Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität, D-44801 Bochum, Germany. Received 3 November 2010.

Keywords: Proxies, brachiopods, conodonts, whole rock, Palaeozoic seawater.

AAP Memoir 41 (2011)10

All proxies, except the conodonts, have modern representatives for investigating their original chemistry and equilibrium incorporation of elements and isotopes into, for example, brachiopod shell carbonate (e.g., Lowenstam 1961) or, into carbonate grains (e.g., Milliman 1974). Great difficulty is encountered with conodonts because they lack modern representatives, and chemical evaluations are usually carried out, by analogy, on other phosphatic materials (e.g., fish bone, teeth, scales). Many researchers share this concern, and according to Trotter & Eggins (2006, p. 199) “…interpreting marine fossil bio-apatite geochemistry is…complicated by the lack of studies on extant faunas against which their true significance as geochemical proxies can be gauged.”

Brachiopods are generally widespread and abundant in Phanerozoic rocks, and subject to few environmental restrictions, but lack high biostratigraphic acuity. The group with the greatest biostratigraphic acuity is the conodonts, with a resolution that is superior to that of the other biogenic proxy. In addition, they possess distribution characteristics for superior identification of ambient environmental settings. Despite their ubiquitous nature, chemistry of whole rock is generally limited to δ13C if it is preserved as micrite (e.g., Veizer 1983) or to material with specific and limiting Sr and Mn contents (e.g., Denison et al. 1994). Denison et al. (1994) assumed that whole rock with Sr content of more than 900 ppm and Mn content of less than 300 ppm retained original seawater-Sr isotope values.

The greatest challenge facing all proxies is their capacity to retain an original chemistry after

the vagaries of geological time and diagenesis. Diagenetic alteration starts soon after death of the biogenic proxy and immediately after precipitation of the abiogenic crystals and grains (e.g., Patterson & Walter 1994). This hypothesis of early diagenetic change applies to all three proxies evaluated in this study (e.g., Walter et al. 1993; Ku et al. 1999; Brand 2004; Zazzo et al. 2004). Alteration may proceed in closed to open diagenetic microenvironments and with it, produce a plethora of diagenetic end products with a multitude of chemical signatures (cf. Brand & Veizer 1980). We believe there is no easy way of assessing the degree of alteration of an individual sample without applying as many screening tests as possible to the selected specimens (cf. Brand 2004). Although many authors invoke exclusively hypothetical/theoretical considerations in deciding whether a sample or sample set have preserved their original chemistry (e.g., Joachimski & Buggisch 2002; Saltzman 2005), others use a multitude of screening tests to reach conclusions about the diagenetic integrity of their sample set (e.g., England et al. 2006; van Geldern et al. 2006). Conodonts with their francolite mineralogy are usually deemed superior in preservation potential to that of brachiopod low-Mg calcite (e.g., Wenzel et al. 2000; Trotter et al. 2008; Joachimski et al. 2009). This assertion is based on two assumptions: 1) that cF is diagenetically more resistant than bLMC; and 2) that oxygen isotopes of conodonts are heavier than those of coeval brachiopods, and by virtue of ‘more’ realistic palaeotemperatures are the better proxy. However, to determine whether they contain an original chemistry is difficult because of the uncertainty of their mineralogical and physical resistance to diagenetic alteration, and a lack of modern counterparts. After several decades, the issue of their preservation potential remains unresolved, for example Joachimski et al. (2004, p. 543) “…assumed… conodonts…to have a high preservation potential…”, whereas Trotter & Eggins (2006, p. 213) “…consider tissue ultrastructure and sample permeability in the context of diagenesis…”, and Iacumin et al. (1996, p. 1) state that “…carbonate fluorapatite is the major mineral phase of apatitic fossils and is considered a replacement phase of the original hydroxylapatite…”. This involves a chemical change in hydroxylapatite [Ca5(PO4)3(OH)]to francolite [Ca10(PO4)6-x(CO3)xF0.4xF2] and x<1.5 with complex substitutions of CO2-3 and other ions and complex ions (cf. Schuffert et al. 1990).

Whole rock, especially fine-grained material, is generally deemed to have preserved its carbon isotopic composition (e.g., Saltzman 2005), despite the observations made in modern

ProxyFig. 1

PC

ICEHOUSE

CAM. ORD.

Aragonite

Aragonite seas

?

Threshold

SIL. DEV. CARB. PER. TR.

GREENHOUSE ICEHOUSE

Calcite seas

Figure 1. Secular variation of non-biogenic marine carbonate mineralogy with geological time (Palaeozoic; modified from Sandberg, 1983; Bates & Brand, 1990). The small perturbations in the first-order cycle represents new information on climatic changes during these time slices (e.g., Isbell et al. 2003) Question mark indicates uncertainty in mineralogical assignment and shift in oscillations. Note: PC – Precambrian, CAM - Cambrian, ORD – Ordovician, SIL – Silurian, DEV – Devonian, CARB – Carboniferous, PER – Permian, TR – Triassic.

AAP Memoir 41 (2011) 11

environments of the impact of syndepositional diagenesis on fine-grained carbonates (e.g., Hover et al. 2001; Walter et al. 2007). The current literature suggests that all and none of the aforementioned proxies are able to retain original seawater chemistry, and the debate rages on as to whether brachiopods, conodonts or whole rocks, make the best proxy.

The objective of this study is to review all theoretical concepts and empirical screening parameters and determine which is the best proxy for identifying original seawater chemistry. The examination will consider brachiopods, conodonts and whole rocks as seawater proxies. Some case studies will be examined to evaluate the reliability of the screening tests and their veracity in determining an ‘original’ seawater chemistry by the selected proxies.

THEORETICAL BACKGROUNDBrachiopodsBrachiopods may secrete calcitic or phosphatic shells, but the sporadic occurrence and ease of diagenetic alteration of their original chemistry eliminates the phosphatic brachiopods as proxies of original seawater chemistry. In contrast,

brachiopods that secrete shells of biogenic low-Mg calcite are favoured by some, because this calcite phase is considered diagenetically more resistant than coeval intermediate-Mg calcite (IMC), high-Mg calcite (HMC) or aragonite (ARAG) (Fig. 2; e.g., Brand & Veizer 1980; Al-Aasm & Veizer 1982; Popp et al. 1986a). Although, their bLMC is resistant to alteration it is not impervious to it, and under the right microenvironmental conditions, such as an open system with high water/rock ratio the bLMC may be altered to diagenetic low-Mg calcite (dLMC), replaced by other minerals (Fig. 3; e.g., Brand 1994; 2004) or completely removed from the host rock/sediment. Most brachiopod bLMC experiences some alteration, but with many occurring in diagenetic microenvironments with low to medium water/rock ratios, which may be reflected in dissolution pits and/or other superficial features but without replacing the original mineralogy and chemistry of the carbonate fibres and prisms (if present) of the secondary and/or tertiary layers (Fig. 4; e.g., Al-Aasm & Veizer 1982; Popp et al. 1986a; Wenzel et al. 2000; van Geldern et al. 2006).

The primary layer is precipitated in disequilibrium with ambient seawater and this material should not be included in isotopic analyses (e.g., Carpenter & Lohmann 1995; Parkinson et al. 2005). Calcite of the secondary and tertiary shell layers is precipitated in isotopic equilibrium with ambient seawater and records ambient water temperature, and is therefore

ProxyFig. 2

8.80 10

LMC IMC HMC

20Mole % MgCO3

- log

IAP

Mg-

calc

ite

30

8.6

8.4

8.2

8.0

7.8

7.6

7.4

7.2

7.0

Biogeniccalcite

Biogenic“Best Fit”

Syntheticcalcite

Aragonite

Figure 2. Stability trends of various marine carbonates (natural, biogenic and synthetic) with changing amounts of magnesium carbonate (modified from Morse & Mackenzie 1990; fig. 3.7). The LMC (low-Mg calcite), IMC (intermediate-Mg calcite) and HMC (high-Mg calcite) ranges are adopted from Brand & Veizer (1980). IAP – ion activity product.

ARAG

O.C.

LMC

dLMC

D.C.

(d)LMC

ProxyFig. 3

High

Water

Flow

Figure 3. Diagenetic open microenvironment system with high water/rock ratio for marine carbonates (modified from Brand & Veizer 1980). ARAG – aragonite, LMC – low-Mg calcite, dLMC – diagenetic low-Mg calcite and (d) probable diagenetic LMC, O.C. – original chemistry, D.C. – diagenetic chemistry. Diagenetic chemistry is controlled largely by the chemistry of the diagenetic fluid and volume exchange.

AAP Memoir 41 (2011)12

suitable as a proxy of original chemistry, if it can be substantiated that the material escaped diagenetic alteration.

ConodontsConodonts consist of apatite and are deemed by some researchers to be resistant to diagenetic alteration, and thus preserve an ambient and original seawater signal (cf. Joachimski et al. 2009). Conodonts consist of francolite (carbonate fluorapatite, Hass & Lindberg 1946; Pietzner et al. 1968; Schuffert et al. 1990), and indeed, are susceptible to thermal alteration (e.g., Epstein et al. 1977) and other diagenetic processes (e.g., Iacumin et al. 1996; Armstrong et al. 2001). It is generally assumed that specimens with a colour alteration index (CAI) of less than 1-2, indicating a burial temperature of less than 80°C, should contain an original seawater chemistry (e.g., Joachimski et al. 2002). However, there is increasing evidence that the carbonate fluorapatite of conodonts and other fossils is the end product of early diagenesis (dissolution-reprecipitation) and not the primary hydroxylapatite, with alteration enhanced by microbial enzymatic activity under natural low-temperature conditions (e.g., Soudry & Champetier 1983; McArthur et al. 1987; Schuffert et al. 1990; Blake et al. 1997; Sharp et al. 2000). Research shows that hydroxylapatite of modern fish, mammals and reptiles and apatitic fossils including carbonate fluorapatite of conodonts contain phosphate and structural carbonate within the apatite lattice, with the latter susceptible to dissolution-reprecipitation processes (e.g., Iacumin et al. 1996), whereas the former’s alteration is mediated by bacteria (e.g., Zazzo et al. 2004; Fig. 5). Other studies

show that different parts (albid crown, hyaline crown, basal body) with their different densities might be subject to different degrees of diagenetic alteration. The evidence is overwhelming in suggesting that conodont cF and their different element components are by no means immune to diagenesis as suggested by some researchers, and the screening of this material is a required necessity in deciding between good and bad proxy material and before using their chemical composition as representing original seawater chemistry (e.g., Kolodny & Luz 1991; Tütken et al. 2008; Bright et al. 2009; Kocsis et al. 2010).

Whole RockWhole Rock encompasses a large degree of textural variation ranging from fine-grained to coarse–grained bioclastic carbonate rocks (e.g., Bathurst 1975). Hypothetically, fine-grained micritic rock is assumed to retain its original seawater-13C composition because: a) preservation of the delicate crystallites of the original rock material infers alteration of the original carbonate sediment in a closed diagenetic microenvironment (e.g., Veizer 1983); and b) in a system with low water/rock ratio (Fig. 4), the chemistry of the diagenetic fluid would be rock buffered, and consequently the δ13C of the diagenetic product would be controlled by that of the original carbonate mineral. Thus, the chemistry of the diagenetic water is considered of secondary importance and of little consequence in determining the chemistry of the diagenetic

ARAG

O.C.

LMC

dLMC

D.C.

LMC

ProxyFig. 4

Low

Water

Flow

Figure 4. Diagenetic, partly closed microenvironment with low water/rock ratio for marine carbonates (modified from Brand & Veizer 1980). Acronyms as in Figure 3. In this system, the diagenetic chemistry is controlled largely by the original carbonate chemistry.

ProxyFig. 5

bA

O.C.

cF

D.C.

Medium

Water

bacteria

Flow

Figure 5. Diagenetic, partly open microenvironment with intermediate water/rock ratio and the presence of enzymatic bacteria for biogenic apatite (modified from Brand & Veizer 1980; e.g., Soudry & Champetier 1983; Iacumin et al. 1996; Sharp et al. 2000; Zazzo et al. 2004). The original chemistry (O.C.) of the biological apatite (bA) is bacterially changed to diagenetic francolite (cF) with its own and distinctive diagenetic chemistry (D.C.).

AAP Memoir 41 (2011) 13

whole rock. However, theory-based preservation of δ13C in the diagenetic whole rock, does not apply universally to all trace elements and other isotopes, both stable and radiogenic. Trace element concentrations in the diagenetic product are significantly different from those in the original precursor material, which show decreases in Sr and Na and concomitant increases in Mn and Fe contents (cf. Brand & Veizer 1980). Similarly, oxygen isotopes are usually more negative in the diagenetic phase, whereas carbon isotopes may be in some special circumstances preserved in what may be deemed an ‘original’ composition (cf. Brand & Veizer 1981; Brand 2004). Carbon isotope value preservation is not unequivocal in carbonate sediment and rock, and it may be seriously compromised during syndepositional diagenesis in the presence of CO2-charged marine pore water leading to both more negative and positive values (cf. Walter et al. 1993; Patterson & Walter 1994; Hu & Burdige 2007). Some texturally preserved lithographic limestone (Louisiana Limestone of Missouri and Illinois) has δ13C values suggesting alteration by CO2-charged meteoric water in a closed to partly closed diagenetic microenvironment (Brand et al. 2004), which warrants a caveat of caution to the wholesale use of micrite-based δ13C. Strontium isotopes of whole rock are used profusely to infer original seawater compositions, and the basis

for this assumption is either a static limit of Sr (>900 ppm) and Mn (<300 ppm) content (e.g., Denison et al. 1994), or the implicit acceptance of the hypothesis of δ13C preservation in fine-grained sample material (e.g., Burke et al. 1982; Bailey et al. 2000). However, the evidence is incontrovertible that not all whole rock samples unequivocally preserve their chemical composition, and thus the geochemical results should be subjected to a number of screening tests to validate their use as original seawater compositions.

SCREENING TESTSOver the last decades a number of screening tests have been presented in the literature to evaluate the veracity of material used as proxies of original seawater chemistry (Table 1).

Visual and Optical ExaminationAmong the multitude of screening tests are: visual and optical (petrographic) inspections of the proxies for obvious signs such as inconsistent colouration, powdery appearance, lack of structural cohesion suggesting impacts of weathering/alteration or to define their textural parameters and changes (e.g., Brand & Veizer 1980; Denison et al. 1994; Brand 2004; Table 1). This type of screening test is easily applied to all three major proxies evaluated in this study, and

Table 1. Summary of screening tests available for the various Palaeozoic proxies (Brachiopods – B; Conodonts – C; Whole Rock – WR). Check mark (√) indicates applicability of screening test to proposed proxy material, and selected source references of application.

 Screening  Test   B   C   WR            Select  References  

Visual,  optical  examination   √   √ √ Lowenstam,  1961; Brand  &  Veizer,  1980;  Popp  et  al.,  1986a  Brand,  2004  

Microstructures   √   √   √   Bates  &  Brand,  1991;  van  Geldern  et  al.,  2006;  Mii  et  al.,  1999;  Zazzo  et  (SEM,  TEM,  EBSD)         al.,  2004;  Griesshaber  et  al.,  2007;  Trotter  et  al.,  2007;  Zaefferer  et  al.,           2008;  Merkel  et  al.,  2009  

Crystallinity  (FT-­‐IR/Raman)     √     Shemesh,  1990;  Nöth  &  Richter,  1992;  Freeman  et  al.,  2001;  Puceat  et  al.,  2004  

Colour  Alteration  Index     √     Epstein  et  al.,  1977;  Armstrong  et  al.,  2001  

Luminescence  (Cathode)   √     √   Rush  &  Chafetz,  1990;  Grossman  et  al.,  1996;  England  et  al.,  2006  

Elemental  Distribution   √   √   √   Brand  &  Veizer,  1980;  Al-­‐Aasm  &  Veizer,  1982;  Shemesh,  1990;  Girard  &  Albarede,  1996;  Armstrong  et  al.,  2001;  Lee  et  al.,  2004;  Brand  2004;  Cusack  et  al.,  2008a  

Stable  Isotope  Distribution   √   √   √   Sharp  et  al.,  2000;  Joachimski  &  Buggisch,  2002;  Auclair  et  al.,  2003;  Brand,  2004;  Zazzo  et  al.,  2004;  Saltzman,  2005;  Korte  et  al.,  2005;  Kolodny  &  Luz,  1991;  Iacumin  et  al.,  1996  

Radiogenic  Isotope  Distribution   √   √   √   Brand,  1991;  Denison  et  al.,  1994;  Ruppel  et  al.,  1996;  Veizer  et  al.,  1997;  Bailey  et  al.,  2000;  Armstrong  et  al.,  2001;  Brand,  2004        

REE  Distribution   √   √   √   Bertram  et  al.,  1992;  Lecuyer  et  al.,  2004;  Trotter  &  Eggins,  2006;  Tütken  et  al.,  2008;  Bright  et  al.,  2009;  Azmy  et  al.,  2010  

Clumped  Isotopes   √   √   √   Ghosh  et  al.,  2006;  Came  et  al.,  2007;  Huntington  et  al.,  2009  

_________________________________________________________________________________________________________________________________________________  

AAP Memoir 41 (2011)14

should be considered a first step in characterising pristine proxy material.

Microstructural ExaminationA more detailed investigation of textures and intrinsic structures of study material may be conducted by scanning electron microscopy (SEM) and/or by transmission electron microscopy (TEM; e.g., Bates & Brand 1991; Samtleben et al. 2001; van Geldern et al. 2006; Trotter et al. 2007; Table 1). The SEM screening test has found wide application in testing brachiopods for pristine features, because of its ease in identifying changes in the delicate nature of both the elongate fibres of the secondary layer and the more robust prisms of the tertiary shell layer (e.g., Merkel et al. 2009, fig. 12). In contrast, few studies examine conodonts by SEM (e.g., Königshof 2003) and even fewer studies use the SEM tool to screen for diagenetic effects in whole rock (cf. Hover et al. 2001). Instead, TEM as a screening tool for conodonts is well suited for characterising material that has good potential of preservation and thus serving as a proxy for original seawater chemistry (e.g., Pierce & Langenheim 1969; Kemp 2002; Trotter et al. 2008). SEM is a powerful screening tool in differentiating well preserved from poorly preserved brachiopod shell material, whereas TEM is the screening tool of choice for conodonts, use of SEM/TEM needs to see greater application in screening for textural changes in whole rock material (cf. Hover et al. 2001).

Electron backscatter diffraction (EBSD) is a facile and automated microdiffraction method suitable for obtaining space-resolved information on the phase-state of biogenic material and on crystallite orientation (Table 1; microtexture; Deal & Eades 2008; Zaefferer et al. 2008). In EBSD, the monochromatic electron beam of an SEM is directed onto a surface point of the sample at an angle of 70°, and a Kikuchi-diffractogram is recorded on a CCD-detector that is indexed automatically. High-resolution microstructure and texture investigations in combination with chemical analyses enable the investigator to decipher overprints (metabolic or diagenetic) effects in both, biogenic and non-biogenic archives (cf. Schmahl et al. 2004, Griesshaber et al. 2007). Overprint processes perturb or eliminate entirely original crystal orientations, and/or disrupt pristine material shapes in conjunction with changing chemical signatures (cf. Brand & Veizer 1980). Greater use of this technique and other screening tests is advisable for all potential proxies.

Cathode Luminescence ExaminationCathode luminescence (CL) has found favour in some quarters as the primary screening test for diagenetic alteration of carbonates (e.g., Popp et al. 1986a; Bates & Brand 1990; Grossman et al. 1996; Table 1). Unfortunately, this test is not an infallible indicator of diagenesis. Instead, luminescence may be observed in fossil and modern biogenic allochems (e.g., brachiopods and cephalopods) and be related to growth rate variation instead of alteration (e.g., Barbin & Gaspard 1995; Tomasovych & Farkas 2005). Or diagenesis may bring about shifts in elemental and isotopic compositions without affecting the luminescence of the carbonate mineral (e.g., Rush & Chafetz 1990; Fig. 3). Instead, the newer, hyperspectral CL imaging process (a combination with spectroscopy) may overcome some of the inherent traditional CL limitations (e.g., England et al. 2006).

Trace Element DistributionThe elemental distribution in carbonates (whole rock and brachiopods) follows specific trends based on meteoric or marine pore water diagenesis, in part governed by the applicable partition coefficients/fractionation factors and the water/rock ratio of the diagenetic microenvironment (e.g., Brand & Veizer 1980; Lee et al. 2004; Cusack et al. 2008b; Table 1). Some investigators use dynamic (e.g., Bates & Brand 1991; Brand 2004) whereas others use static (e.g., Denison et al. 1994) or modern-based (e.g., Korte et al. 2005) limits of trace element concentrations to differentiate between original and altered test material (Table 1). The static limit at times is too inflexible and will not account for natural variations of the ambient environment (cf. Bates & Brand 1991), and it will erroneously categorise material into disparate end-members (cf. Denison et al. 1994; Veizer et al. 1999). A more flexible and dynamic approach is favoured in the interpretation of trace element composition, which is supported by the concentrations measured in modern brachiopods representing many different localities and environments (cf. Morrison & Brand 1986; Brand et al. 2003). Despite a general lack of studies that deal with the elemental composition of conodonts (Wright et al. 1988; Girard & Albarede 1996), a few recent studies demonstrated that tooth enamel might become enriched in elemental concentrations with diagenesis (e.g., Kohn et al. 1999; Trueman & Tuross 2002). The micromapping of elemental distributions in all potential proxies may be an approach to embrace in their examination (cf. Cusack et al. 2008b), such as the high-resolution mapping/distribution of elemental concentrations

AAP Memoir 41 (2011) 15

in brachiopod calcite (e.g., Cusack et al. 2008a; Pérez-Huerta et al. 2008).

Stable Isotope DistributionStable isotopes and their distribution may be an indicator of either depositional or diagenetic effects, and because of this complexity and their use as hydrosphere and climatic indicators some investigators do not use them as screening tools, which in our opinion is an unnecessary deletion of a powerful method (Table 1). Carbon and oxygen are the major isotopes used to differentiate between what may be original values in brachiopods, conodonts, and whole rock (mostly carbon). Sometimes, comparisons are made between organic carbon δ13C and carbonate carbon δ13C, or between proxies such as conodonts and brachiopods (e.g., Diener et al. 1996; Wenzel et al. 2000; Joachimski et al. 2004). Based on these few comparative studies, the oxygen isotope values of brachiopods tend to be more negative than those of the coeval conodonts. This is generally interpreted to suggest that conodonts are better preserved than their calcitic companions (cf. Tudge 1960; Kolodny et al. 1983; Joachimski et al. 2004; Joachimski & Buggisch 2002). However, the early diagenesis of conodont bioapatite to francolite leads to an increase in δ18O values of about 1-2 ppm (SMOW; Fig. 6) and thus to underestimates in the ambient temperature of about 5°C (Sharp et al. 2000). Many empirical studies have demonstrated problems with the assumption of universal preservation of any

proxy (cf. Kolodny et al. 1996). Instead, conodont apatite with its elemental, isotopic and REE composition for the most part are diagenetic signals and the results should be used with great caution (cf. Bright et al. 2009; Kocsis et al. 2010).

Colour Alteration IndexThe colour alteration index (CAI) has limited application in determining the diagenetic integrity of conodont francolite (Armstrong et al. 2001), since it relates the maturation of organic tissue with burial temperature of the conodont element (Table 1; Epstein et al. 1977). This parameter appears to be more closely related to the maturation of the host rock and its potential for bearing hydrocarbons, and thus serves as a hydrocarbon exploration tool. Even at a CAI of < 1.5 the Sr isotopic composition of conodonts is inferior to that of coeval biogenic carbonates (cf. Veizer et al. 1997; Brand et al. 2003), and this concern applies equally to their REE contents (Trotter & Eggins 2006).

REE DistributionThe REE content and distribution has found wide application with whole rock and conodonts, but most of it was from the perspective of ‘original’ values and not as a screening tool (Table 1). The REE determination of brachiopod bLMC is a recent application, and in a preliminary study Azmy et al. (2010) demonstrated that some fossil brachiopods have REE trends similar to those of modern counterparts, whereas altered ones exhibit the typical ‘bell-shaped’ trend observed in whole rocks and conodonts. The application of REE trends and distribution in brachiopods is still in its infancy and more work is required to resolve the issues of applicability to seawater reconstruction. Reynard et al. (1999, p. 435) examined the distribution of REE of fossil apatite, and according to them the bell-shaped patterns “… are best explained by REE fractionation during uptake as the material recrystallises…” and reflects “…extensive late diagenesis and not a primary signal”. Similar results were noted by Bright et al. (2009), who observed enrichment in middle rare earth elements and suggested a diagenetic process for this uptake (cf. Kocsis et al. 2010). Despite, this concern with diagenetic uptake of REEs during diagenesis by conodonts, the albid crown may be more resistant to this process and contain ‘original’ concentrations. However, more extensive work is required to resolve the issue of selective diagenesis in different parts of the conodont element. The REE content of whole rock (carbonate) is not a major consideration due to the problems of diagenesis and elemental redistribution, but biogenic

δ18

O (‰

, SM

OW

)

δ13C (‰, PDB)

ProxyFig. 6

-8 -6 -4 -2 0

28

24

20

dentine

enamelShark tooth(Miocene)

Diagenetic

alteration

Figure 6. Stable isotope compositions in different apatite materials of teeth of Miocene sharks suggest differential diagenetic alteration (modified from Sharp et al. 2000). Enamel of teeth is more resistant to alteration than dentine, both with a major mineralogy of diagenetic carbonate fluorapatite (francolite; e.g., Iacumin et al. 1996).

AAP Memoir 41 (2011)16

carbonates, other than brachiopods, have found only minor application in seawater reconstruction studies (e.g., Elderfield et al. 1981; Banner et al. 1988; Webb & Kamber 2000).

Clumped Isotope DistributionThe clumped isotope (∆47) method of carbonates is a new tool that uses isotopologues that are independent of ambient water composition, and thus reflect the temperature of the coeval environment (Table 1). This is based on the temperature-based ordering of 13C and 18O in the carbonate lattice instead of the bulk δ13C and/or δ18O values, which are both water temperature and composition dependent (Ghosh et al. 2006). In a recent application to fossil material (brachiopods and molluscs), Came et al. (2007) determined that diagenetically altered material gave temperatures that were unrealistic compared to the preserved one that contained an original seawater signal. The use of clumped isotopes as a screening tool is currently not a feasible proposition due to its high cost, time-consuming analysis and required large sample weight.

Radiogenic Isotope DistributionRadiogenic isotopes, specifically strontium, are valuable tools in screening good proxy material from bad (Table 1). With other pre-screening tools (Table 1), strontium isotope results should show an increase in radiogenic values with increasing diagenetic alteration (Elderfield 1986; Veizer 1989; Brand 1991). Despite the theoretical predictions of changes with diagenesis, many studies using whole rocks or conodonts as proxies of original seawater composition failed to apply rigorous screening tests as to their preservation (e.g., Kovach 1981; Burke et al. 1982). Bertram et al., (1992) determined that conodont basal body francolite may be prone to diagenesis, but otherwise conodont elements with a CAI of < 2.5 and only phosphate-associated Sr should have preserved their original seawater chemistry (cf. Armstrong et al. 2001). Modifications were made over the years in the preparation method of whole rocks to avoid the production of erroneous results (Bailey et al. 2000). A recent evaluation of various Palaeozoic carbonate material, demonstrated that whole rock 87Sr/86Sr values despite careful attention to the dissolution process and acid, are invariant and tend to follow diagenetic trends (Brand 2004). Similarly, the strontium isotopic content of conodonts and brachiopods may change with degrees of diagenetic alteration including meteoric as well as hydrothermal/marine fluids (e.g., Brand 1991; Brand et al. 2010), unless additional pre-screening and or comparative results confirm the pristine nature

of the studied proxy.

Crystallinity ExaminationNöth & Richter (1992) explored the use of Fourier Transform infrared spectroscopy (FT-IR) for assessing changes in the crystallinity of conodont francolite from field and heating experiments (Table 1). They observed a gradual but steady increase in the carbon dioxide and decrease in the water content of conodont francolite with increasing thermal diagenesis (Nöth & Richter 1992, figs. 7, 8). Their evidence was conclusive that chemical changes occurred in conodonts with thermo-diagenesis. Similarly, Shemesh (1990) used FT-IR crystallinity index (CI) as a screening tool of preservation potential in sedimentary apatites (Table 1). He determined by infrared spectroscopy that changes in elemental and isotopic compositions are related in part to carbonate ions substituting for OH- and PO3-

4 in the apatite lattice (e.g., Shemesh 1990). Shemesh (1990, p. 2435) further determined, based on crystallinity indices, that “…all the Palaeozoic phosphorites …are recrystallized.” He determined that the degree of crystallinity was not related to geological age but instead that environmental conditions such as accumulation rate as well as pore water volume and chemistry governed the recrystallisation process, and increasing crystallinity corresponded to decreases in Sr content and less negative δ18Op (SMOW) isotope values (Shemesh 1990, figs. 7, 8). These observations suggest that CIFT-IR makes for a good screening tool of conodont apatite preservation. However, in a recent study of similar materials using a similar but faster screening tool (Raman spectroscopy; CIRaman), Puceat et al. (2004) encountered disappointing results. The CIRaman and the coeval isotopic and REE redistribution of apatitic materials did not always accord with degrees of diagenetic alteration (Puceat et al. 2004, fig. 7), and the authors suggested that the Crystallinity Index (limited to CIRaman) does not adequately discriminate between pristine and diagenetic samples.

CASE STUDIESA few case studies will be examined to demonstrate that no individual screening test is a panacea in finding or defining the best proxy material/sample for reconstructing original seawater compositions. Instead, from the above and the case studies it becomes quite clear that several screening tests are needed to effectively identify the material with the most probable original seawater composition. Case Study 1 – Miocene Shark TeethDetailed examination of biological apatite makes

AAP Memoir 41 (2011) 17

it clear that francolite is an early diagenetic product mediated by bacterial enzymatic activity of original hydroxylapatite (e.g., Iacumin et al. 1996; Kolodny et al. 1996; Longinelli 1996; Blake et al. 1997). Furthermore, these and other studies suggest that during this process, oxygen isotope values are shifted by about +1 to +2‰ (cf. Sharp et al. 2000), but that some parts of the carbonate fluorapatite are more resistant to further diagenetic alteration than others. Indeed, enamel apatite is more resistant than coeval dentine apatite to subsequent diagenetic attack, which brings about significant shifts in both oxygen and carbon isotopes. In the case of shark teeth from the Miocene, their δ18O (SMOW) values are shifted towards more positive values by about 1 to 6‰, whereas their δ13C values become more negative by about 1 to 4‰ with progressive diagenesis (Fig. 6). The study by Sharp et al. (2000) and those of others (e.g., Zazzo et al. 2004) clearly demonstrate that biological apatite and thus conodonts are not a panacea for obtaining original seawater chemistry concentrations.

Case Study 2 – Louisiana LimestoneA lithographic limestone and coeval brachiopod was part of an investigation into the Devonian-Carboniferous boundary, and some of the material was evaluated using visual information, microstructures and trace chemistry, with supplementary evaluation of their carbon, oxygen and strontium isotope composition (Fig. 7; Brand et al. 2003). The lithographic limestone sample was from the Louisiana Limestone

and according to theoretical considerations, its fine-grained texture should have preserved its original carbon isotopic composition. The coeval brachiopods, based on several screening tests (visual, microstructures, trace chemistry) were

7.0

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etic a

lterat

ion

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BrachiopodsLatest Devonian

ProxyFig. 7

0.70810 0.70815 0.70820 0.70825 0.7083087Sr/86Sr

Figure 7. Carbon, oxygen and strontium isotopic compositions of lithographic limestone and coeval brachiopod calcite (from Brand et al. 2003). Diagenesis of the fine-grained whole rock, which according to theory should have retained its original δ13C value, shows a significant shift towards more negative values than the chemically and microstructurally well preserved coeval brachiopod bLMC. At the same time the 87Sr/86Sr is the most radiogenic in the whole rock sample and well outside the range of natural variation (cf. Brand et al. 2003).

Figure 8. Diagenetic results and trends are complicated by subsequently superimposed dolomitisation processes for some carbonate allochems from the Bird Spring Formation (cf. Brand & Brenckle 2001). The dolomitised whole rock has δ13C values that are similar and δ18O values that are more positive than those of coeval non-dolomitised brachiopod samples. Vein calcite cement reflects the typical negative carbon and oxygen isotope values of meteoric water post-depositional alteration (Brand & Veizer 1981).

δ13

C (‰

, PD

B)

δ18O (‰, PDB)

ProxyFig. 8

Vein Cement

alteredbrachiopods

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AAP Memoir 41 (2011)18

judged to be preserved in their original bLMC, and thus their δ13C values should be similar to those of the whole rock. Contrary to theoretical considerations espoused for the carbon isotopic composition of whole rock, the fine-grained lithographic Louisiana Limestone has more negative δ13C, δ18Ο and more radiogenic 87Sr/86Sr values, all indicative of extensive alteration in a partly open diagenetic system in contact with meteoric water (cf. Brand 2004). Thus, adherence to strictly theoretical concepts without follow-up screening and verification may lead to erroneous results and interpretations.

Case study 3 – Bird Spring FormationIn another case study, the whole rock sample is similar in carbon isotopes and more positive in oxygen isotopes than those of coeval and altered brachiopods (screened by visual inspection,

trace chemistry, microstructures) from a horizon of the Bird Spring Formation, Nevada (Fig. 8; Brand & Brenckle 2001). This concurrence of δ13C values would suggest that both proxies are well preserved and have retained an original seawater composition. Instead, the δ13C and δ18O values and trends are explained in terms of enrichment in both isotopes through the process of dolomitisation (Fritz & Smith 1970), and thus neither of them (brachiopods and whole rocks) from this particular horizon of the Bird Spring Formation represents original seawater chemistry.

Case Study 4 – Devonian/Carboniferous boundary/Akiyoshi LimestoneThe Devonian-Carboniferous boundary study afforded the opportunity to directly compare the Sr isotope values of the three proxies of interest: brachiopods, conodonts and whole rocks (Fig. 9; Brand et al. 2003). In all instances, the Sr isotope values are more radiogenic in the whole rock and conodont samples, despite the ‘assertion’ that conodonts should preserve and contain an original seawater chemistry (see Theoretical section), and that whole rock, by virtue of its fine-grained texture and alteration in a closed diagenetic microenvironment (see Theoretical section), should also contain an original seawater signal.

In contrast, the brachiopod and whole rock

ProxyFig. 9

0.7080 0.7082 0.7084 0.7086

BrachiopodsConodontsWhole Rock

354 Ma

87Sr/86Sr

DE

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NIA

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enni

an

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erM

iddl

eU

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ulca

ta

Tour

nais

ian

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BON

IFER

OU

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raes

ulca

ta

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Figure 9. Strontium isotopic composition of correlative brachiopod, conodont and whole rock results from the Devonian-Carboniferous boundary (modified from Brand et al. 2004). The 87Sr/86Sr values define a narrow band with a small radiogenic perturbation in the upper S. praesulcata Subzone, similarly the results of the conodonts also define a narrow trend, but one that reflects diagenetic alteration (more radiogenic values than those of the brachiopods), whereas the results of the whole rock are characterised by high variability and stratigraphic uncertainty (Denison et al. 1997).

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δ18O (‰, PDB)87

Sr/86

Sr

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Diag

enet

ic al

tera

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AlteredBrachiopods

Brachiopods

Whole Rock

Figure 10. Alteration in the presence of hydrothermal/marine fluid mixture leads to less radiogenic values in the diagenetic products from the Akiyoshi Formation (modified from Brand et al. 2009, 2010). In this instance, the visually, microstructurally and chemically best-preserved brachiopod bLMC (in the shaded field) is the most radiogenic, contrary to conventional trends.

AAP Memoir 41 (2011) 19

material from Japan (Brand et al. 2009, 2010) suggests a complex diagenetic history (Fig. 10). If theoretical considerations prevailed, particularly those related to Sr isotopes, then the whole rocks and brachiopods (solid symbols) would represent the best preserved material and the samples in the shaded box would be deemed altered. However, based on visual inspection, micro- and ultrastructural evaluation, trace chemistry trends and stable isotope distribution, the brachiopods within the shaded box represent the proxy material that has preserved original seawater chemistry. In this instance, diagenesis lead to less radiogenic Sr values in the progressively more altered samples in the presence of hydrothermal/marine fluids (cf. Fig. 11).

Natural VariationMany studies, due to fiscal, sampling or other constraints only present one sample and chemical result per sample horizon. Furthermore, they construct trends based on not only singular samples but on proxies from epeiric sea settings to represent variations of the global ocean, when there is convincing and overwhelming evidence both from the ancient record and modern environments that carbonate platforms are not universally representative of open ocean conditions (e.g., Walter et al. 1993; Patterson & Walter 1994; Holmden et al. 1998; Brand et al. 2009). The carbon and oxygen isotope results of modern brachiopods from Palau clearly demonstrate the need to collect sufficient material to ‘capture’ the natural variation of the local environment (Fig. 12; Carpenter & Lohmann 1995), anything less may lead to serious

misinterpretations (cf. Bates & Brand 1991; Curry & Fallick 2002). This could be achieved by testing several specimens/samples or by sampling along the shell’s growth axis (cf. Lee et al. 2004). Without due consideration for natural variation, the observed variations of 1.2‰, δ18O and 1.8‰, δ13C would or could be interpreted as changes in climate (temperature) and carbon burial, respectively, which of course in this instance are completely erroneous. Thus, we recommend that as many samples as possible should be analysed by researchers to satisfy the potential of local and regional natural environmental variation (cf. Brand et al. 2004).

SUMMARYThis review of the three major proxies (brachiopods, conodonts, whole rocks) for the Palaeozoic rock record, is far from exhaustive, but provides sufficient insight that diagenesis and the alteration of textures, internal structures, trace elemental contents, stable and radiogenic isotopic compositions may cause everything from subtle to distinct and observable changes.

It is not the material with the best theoretical concept, nor the material that is the most readily available, nor the material that is the easiest to analyse that makes the best proxy of original Palaeozoic seawater chemistry.

Instead, we propose that the best proxy is the one that has passed as many of the applicable screening tests listed in Table 1 and compares favourably with coeval material. The best proxy is the one that provides as much stratigraphic

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0.70810 Hydrothermaldiagenesis

0.70805

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0.707956 6

CAI5 2

87S

r/86S

r

DY

KE

ProxyFig. 11

Figure 11. Hydrothermal alteration of conodonts (based on colour alteration index - CAI; Epstein et al. 1977) from near a dyke leads to less radiogenic 87Sr/86Sr values (modified from Armstrong et al. 2001). This is similar to Sr isotope trends observed in hydrothermally altered whole rocks and brachiopods from the Akiyoshi Formation (cf. Fig. 10).

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, PD

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ProxyFig. 12

1.0

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Figure 12. Carbon and oxygen isotopic compositions of modern brachiopods from Palau (modified after Carpenter & Lohmann 1995). The dashed (majority) and dotted (all samples) fields encompass the total natural variation observed/measured in shell calcite of the primary layer of Terebratella sp. (cf. Bates & Brand 1991; Curry & Fallick 2002).

AAP Memoir 41 (2011)20

information as possible to assure the high-resolution age assignment so often missing from studies of ancient sequences. The best proxy is the one with a sufficiently large database for each sample horizon to take account of the normal and expected natural variation so prevalent in carbonate platforms in modern epeiric seas and open oceans. Lastly, all geochemical and morphological information should be well documented and readily available to the reader for critical examination of the results.

ACKNOWLEDGEMENTSWe thank M. Lozon for drafting the figures and plates, and G. Hooper for SEM work. The national research councils of Canada, Germany and Poland provided financial support to the respective authors. The reviewers H. -S. Mii and M. Cusack, and the editor E. A. Weldon are thanked for their input that made this article more concise.

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