Use of multiple oxygen isotope proxies for elucidating Arctic Cretaceous palaeo-hydrology

Stable oxygen isotope analysis of siderite and dinosaur tooth enamel phosphate from the Campanian–Maastrichtian Prince Creek Formation, Alaska, USA, are analysed to determine the palaeohydrology of the ancient Colville Basin north of the Ancestral Brooks Range. δ¹⁸O of freshwater siderites relative to V-PDB ranges between −14.86 and −16.21‰. Dinosaur tooth enamel δ¹⁸O from three different sites (Kikak–Tegoseak, Pediomys Point, Liscomb) range between +3.9‰ and +10.2.0‰. δ¹⁸O_{meteoric water} are calculated from δ¹⁸O_siderite that formed at seasonal temperatures ranging from −2 to 14.5 °C, with a mean annual temperature of 6.3 °C. At 6.3 °C, the δ¹⁸O_w calculated from siderite ranged between −22.23 and −20.89‰ V-SMOW. Ingested water compositions are estimated from dinosaur teeth assuming body temperatures of 37 °C and local relative humidity of 77.5%, resulting in values ranging from −28.7 to −20.4‰ V-SMOW, suggesting consumption of meteoric water and orographically depleted runoff from the Brooks Range. The ranges in calculated δ¹⁸O_{meteoric water} are compatible between the two proxies, and are mutually corroborating evidence of extremely ¹⁸O-depleted precipitation at high latitudes during the Late Cretaceous relative to those generated using general circulation models. This depletion is proposed to result from increased rainout effects from an intensified hydrological cycle, which probably played a role in sustaining polar warmth.

本研究对美国阿拉斯加州坎潘期（Campanian）-马斯特里赫特期（Maastrichtian）王子溪组（Prince Creek Formation）中的菱铁矿（siderite）与恐龙牙齿釉质磷酸盐开展稳定氧同位素分析，旨在解析始祖布鲁克斯山脉（Ancestral Brooks Range）以北古科维尔盆地（Colville Basin）的古水文环境。相对于维也纳佩迪多贝箭石标准（V-PDB），淡水菱铁矿的δ¹⁸O值介于-14.86‰至-16.21‰之间。来自Kikak–Tegoseak、Pediomys Point与Liscomb三个采样点的恐龙牙齿釉质δ¹⁸O值介于+3.9‰至+10.2‰之间。研究人员以形成于-2℃至14.5℃季节性温度区间、年均温6.3℃环境下的菱铁矿δ¹⁸O值为基础，计算得到大气降水δ¹⁸O（δ¹⁸O_meteoric water）值。在年均温6.3℃条件下，由菱铁矿计算得到的水体δ¹⁸O（δ¹⁸O_w）值介于-22.23‰至-20.89‰（维也纳标准平均海水，V-SMOW）之间。研究以恐龙牙齿为载体，假设其体温为37℃、当地相对湿度为77.5%，估算出其摄入水体的δ¹⁸O值介于-28.7‰至-20.4‰（V-SMOW）之间，表明该恐龙摄入的是源自布鲁克斯山脉地形贫化径流的大气降水。两种代用指标得到的大气降水δ¹⁸O值范围具有一致性，二者共同证明，晚白垩世高纬度地区的降水具有极强的¹⁸O贫化特征，这一结果与通用环流模型（general circulation models）的模拟结果存在差异。这种¹⁸O贫化现象被认为是由增强的水文循环带来的更强降雨洗脱效应所导致，而该效应可能在维持极地温暖气候方面发挥了重要作用。