Effect of diagenesis on the Sr, O, and C isotope composition of late Cretaceous mollusks from the Western Interior Seaway of North America

Abstract

Evaluating the effects of diagenesis on the isotopic compositions of Sr, O, and C in marine carbonates is critical to their use as proxies in reconstructing information on the salinity, temperature and dissolved inorganic carbon of ancient oceans. We have analyzed a series of samples of mollusk shells from the Baculites compressus zone (late Campanian) of the Pierre Shale of South Dakota. Samples included outer shell material and septa of cephalopods collected inside and outside concretions. Preservation was evaluated using light microscopy, scanning electron microscopy (SEM), trace element analysis and X-ray diffraction. All of the material consists of aragonite based on X-ray diffraction. An SEM preservation index (PI) was established based on comparison of the microstructure of the fossil material with that of modern Nautilus. Excellent preservation (PI ‫؍‬ 5) was characterized by well-defined nacreous plates with discrete, angular boundaries. In contrast, samples showing fused nacreous plates with indistinct boundaries were rated poor (PI ‫؍‬ 1). 87 Sr/ 86 Sr ratios vary with preservation and average 0.707648 ؎ .000021 (n ‫؍‬ 10) for excellent preservation (PI Ϸ 5), 0.707615 ؎ .000028 (n ‫؍‬ 5) for good preservation (PI Ϸ 3), 0.707404 ؎ .000074 (n‫)7؍‬ for fair preservation (PI Ϸ 2), and 0.707261 ؎ .000053 (n‫)8؍‬ for poor preservation (PI Ϸ 1). These data suggest that as the quality of the preservation declines, the mean 87 Sr/ 86 Sr ratio decreases and the standard error of the mean increases. Oxygen and carbon isotope analyses of the same specimens also show decreases with preservation, and ␦ 18 O, ␦ 13 C and 87 Sr/ 86 Sr are well correlated, suggesting that these tracers are all altered as the PI decreases. The Sr/Ca ratio increases as preservation decreases, indicating that Sr is added to the shell material during diagenesis. In contrast, Mg/Ca shows no trend with preservation. If the increasing Sr concentration (and decreasing 87 Sr/ 86 Sr) of the shell material with decreasing preservation represents the addition of Sr to the shell during diagenesis, we calculate that the added Sr had 87 Sr/ 86 Sr ranging from 0.707582 to 0.707032. Potential sources of the added Sr include older marine carbonates and weathering of volcanic ash layers present in the shale. The mechanisms of alteration likely include epitaxial growth of strontianite on the original shell aragonite and isotopic exchange of C and O between alteration fluids and shell carbonate. We conclude that SEM preservation criteria are effective in screening shell material that records original isotopic values and that variations in Sr, O and C isotope composition in well-preserved material can be used to assess paleoenvironmental parameters, such as salinity and temperature. Our results also indicate that assessing preservation is a critical prerequisite to the determination of numerical ages of shell material using strontium isotope stratigraphy.

Ammonoid Habitats and Life History

“…al. 1977;Marshall 1981;Landman et al 1983;Whittaker et al 1987;Fatherree et al 1998;Cochran et al 2003Cochran et al , 2010aMoriya et al 2003;He et al 2005;Ifrim and Stinnesbeck 2010;Henderson and Price 2012;Zakharov et al 2005Zakharov et al , 2006Zakharov et al , 2001Zakharov et al , 2012. Similar investigations were made on the endocochleate belemnites (Lowenstein and Epstein 1954;Spaeth et al 1971;Ditchfield 1997;Podlaha et al 1998;Price et al 2000;Niebuhr and Jochiamski 2002;McArthur et al 2004;Price and Mutterlose 2004;Dutton et al 2007;Zakharov et al 1999Zakharov et al , 2005Zakharov et al , 2011.…”

“…Besides these morphological and sedimentological considerations, stable isotope data (δ 13 C, δ 18 O, 87 Sr/ 86 Sr; clumped isotopes) extracted from original shell material (i.e., aragonite) of ammonoids could yield new insights in ammonoid lifestyle, ontogeny and habitats (Jordan and Stahl 1970;Brand 1986;Anderson et al 1994;Fatherree et al 1998;Tsujita and Westermann 1998;Cochran et al 2003Cochran et al , 2010aMoriya et al 2003;Lukeneder et al 2010;Dennis et al 2013;. If we can neglect vital effects in calcification of ammonoid (Urey et al 1951;Moriya et al 2003) and Recent Nautilus shells, then the physicochemical properties, especially δ 18 O, would provide independent and solid evidence for calcification temperature (and salinity).…”

Scaphites of the “Nodosus Group” from the Upper Cretaceous (Campanian) of the Western Interior of North America

“…In the concretions from the Baculites compressus-B. cuneatus zones of the Pierre Shale in western South Dakota and east-central Montana, most specimens retain their original aragonitic shell structure, although the quality of preservation varies (Cochran et al, 2010). In contrast, in the sandstone concretions from the B. compressus-B.…”

“…The authors used X-ray diffraction analysis to determine the suitability of these samples for isotopic study. However, Cochran et al (2010) have argued that even material that is still mostly aragonite may have neverthess undergone alteration, yielding erroneous isotopic values. They proposed a screening protocol emphasizing scanning electron microscopy to assess the state of preservation of the nacre.…”

Diagenetic alteration in low-Mg calcite from macrofossils: a review

“…The pres er va tion of the lat tice struc ture of sin gle crys tal fibres can be con trolled us ing elec tron back scat ter diffractometry (EBSD; e.g., Cusack and Freer, 2008;Pérez-Huerta and Cusack, 2009). It is also pos si ble to estab lish pres er va tion in di ces on the ba sis of SEM im ag ery that can be com pared to geo chem i cal data gen er ated for the fos sil spec i mens (Cochran et al, 2010;Jelby et al, 2014). In many instances, only a lim ited num ber of rep re sen ta tive sam ple fragments can be checked us ing the SEM tech nique rather than the com plete set of aliquots which are taken to de lin eate geo chem ical trends.…”