An assessment of latest Cretaceous Pycnodonte vesicularis (Lamarck, 1806) shells as records for palaeoseasonality: A multi-proxy investigation, link to supplementary data

"In order to assess the potential of the honeycomb oyster Pycnodonte vesicularis for the reconstruction of palaeoseasonality, several specimens recovered from the late Maastrichtian Neuquén Basin (Argentina) were subject to a multi-proxy investigation, involving scanning techniques, trace element and isotopic analysis. Combined CT scanning and light microscopy reveals two major calcite micromorphologies in P. vesicularis shells (vesicular and foliated calcite). Micro-XRF analysis and cathodoluminescence microscopy show that reducing pore fluids were able to migrate through the vesicular portions of the shells (aided by bore holes) and cause recrystallization and precipitation of secondary carbonate in the porous micromorphology, thus rendering the vesicular portions not suitable for palaeoenvironmental reconstruction. In contrast, stable isotope and trace element compositions show that the original chemical composition of the shell is well-preserved in the denser, foliated portions, which can therefore be reliably used for the reconstruction of palaeoenvironmental conditions. Stable oxygen and clumped isotope thermometry on carbonate from the dense hinge region yield sea water temperatures of 11°C, while previous TEX86H palaeothermometry yielded much higher temperatures. The difference is ascribed to seasonal bias in the growth of P. vesicularis, causing warm seasons to be underrepresented from the record, and TEX86H palaeothermometry being potentially biased towards warmer surface water temperatures. Superimposed on this annual mean is a seasonality in d18O of about 1 per mil, which is ascribed to a combination of varying salinity due to fresh water input in the winter and spring season and a moderate temperature seasonality. Attempts to independently verify the seasonality in sea water temperature by Mg/Ca ratios of shell calcite are hampered by significant uncertainty due to the lack of proper transfer functions for pycnodontein oysters. The multi-proxy approach employed here enables us to differentiate between well-preserved and diagenetically altered portions of the shells and provides an improved methodology for reconstructing palaeoenvironmental conditions in deep time. While establishing a chronology for these shells was severely complicated by growth cessations and diagenesis, cyclicity in trace elements and stable isotopes allowed a tentative interpretation of the potential annual seasonal cycle in the late Maastrichtian palaeoenvironment of the Neuquén basin. Future studies of fossil ostreid bivalves should target dense foliated calcite rather than sampling bulk or vesicular calcite. Successful application of clumped isotope thermometry on fossil bivalve calcite in this study indicates that temperature seasonality in fossil ostreid bivalves may be constrained by the sequential analysis of well-preserved foliated calcite samples using this method.

为评估蜂窝牡蛎（honeycomb oyster）泡状拟牡蛎（Pycnodonte vesicularis）在古季节性重建中的应用潜力，本研究对产自阿根廷晚马斯特里赫特阶（late Maastrichtian）内乌肯盆地（Neuquén Basin）的多件化石标本开展了多代理（multi-proxy）分析，涵盖扫描技术、微量元素与同位素测试。结合CT扫描（CT scanning）与光学显微镜观察发现，泡状拟牡蛎壳体中存在两种主要的方解石（calcite）微观形貌：泡状方解石与叶状方解石。微X射线荧光（micro-XRF）分析与阴极发光（cathodoluminescence）显微镜观测显示，还原性孔隙流体可通过壳体的泡状区域（借助钻孔作用）运移，并在多孔微观形貌中引发重结晶（recrystallization）作用与次生碳酸盐（secondary carbonate）沉淀，因此泡状区域不适用于古环境重建（palaeoenvironmental reconstruction）。与之相反，稳定同位素（stable isotope）与微量元素（trace element）组成表明，壳体的原始化学组成在更为致密的叶状区域中得到了良好保存，因此该区域可被可靠用于古环境条件重建。对致密铰合区域碳酸盐开展的稳定氧同位素与团簇同位素（clumped isotope）测温结果显示，当时的海水温度为11℃，而此前基于TEX86H的古温标法（palaeothermometry）结果则显著更高。该温度差异被归因于泡状拟牡蛎生长过程中的季节偏差：暖季在记录中占比不足，且TEX86H古温标法可能偏向于估算更高的表层海水温度。叠加于年平均温度之上的是约1‰的δ¹⁸O季节波动，这一现象可归因于冬春季节淡水输入导致的盐度变化与适度的温度季节变化共同作用的结果。尝试通过壳体方解石的Mg/Ca比值独立验证海水温度季节变化的研究，因缺乏针对拟牡蛎类（pycnodontein oysters）的合适转换函数（transfer functions），面临显著的不确定性阻碍。本研究采用的多代理分析方法，可有效区分壳体中保存完好与成岩改造的区域，并为深时（deep time）古环境重建提供了更完善的方法论。尽管壳体的年代学构建因生长停滞与成岩作用（diagenesis）而极为复杂，但微量元素与稳定同位素的周期性特征，使得我们可对内乌肯盆地晚马斯特里赫特阶古环境中的潜在年度季节周期做出初步解释。未来针对古牡蛎类（ostreid bivalves）化石的研究，应优先选取致密的叶状方解石，而非对整体壳体或泡状方解石进行采样。本研究成功在化石双壳类方解石中应用团簇同位素测温技术，表明通过对保存完好的叶状方解石样品开展序列分析，可限定古牡蛎类化石的温度季节变化特征。