Abstract Oceanic Anoxic Event 2 (OAE-2) occurred at the Cenomanian-Turonian boundary (∼94.1 Ma) and was a time of profound global changes in ocean chemistry and the carbon cycle. This event was characterized by a positive carbon isotope excursion (CIE) caused by massive organic carbon burial, global greenhouse temperatures, ocean deoxygenation, and changes in ocean life driven by large igneous province (LIP) activity. LIPs throughout the Phanerozoic have had dynamic magma flux, with episodes of major eruptions interspersed with periods of relatively less intense eruptions. A possible trigger for LIP activity throughout the Phanerozoic has been attributed to extraterrestrial impacts because there are multiple contemporaneous occurrences of large craters, LIP activity, and mass extinctions in the geologic record. At the Cenomanian-Turonian boundary, there is a 25 km diameter (rim-to-rim) complex crater in NW Alberta, Canada known as the Steen River impact structure dated at 91 ± 7 Ma (Carrigy and Short, 1968). An alternative explanation for those craters found contemporaneous with LIP activity and mass extinctions is that they were created by large explosive events related to LIP activity. Explosive events associated with mantle plume incubation beneath cratonic lithosphere have been suggested to create geologic features commonly attributed to impacts (e.g., shocked quartz, microspherules, etc.). Currently, the trigger for LIP activity during OAE-2, as well as the duration of LIP activity and the temporal variation and magnitude of eruption rates are not well constrained. To address the issue of LIP eruption rates and the trigger for LIP activity, we examined osmium (Os) abundances and isotopes as well as highly siderophile element (HSE; for this study: Re, Ru, Pd, Os, Ir, Pt) abundance data from a continuous sedimentary section spanning OAE-2. The section is from the Eagle Ford Group in the Iona-1 core, deposited in the Cretaceous Western Interior Seaway (KWIS). We found three high Os concentration intervals with mantle-like initial Osi isotope (initial 187Os/188Os) values of ∼0.16. These intervals are interpreted to reflect high-flux LIP magmatic pulses. Between the pulses, lower Os abundances with more radiogenic Osi values of ∼0.20 are observed, which we interpret as low-flux LIP activity between the high-flux periods. This trend of high-and-low flux Os concentration pulses with mantle-like Osi values during the high flux periods is found in another KWIS core (USGS Portland #1) deposited to the north of Iona-1, and in core Deep Sea Drilling Project (DSDP) Site 530 Hole A (hereafter DSDP 530A; drilled off-shore Namibia in the Angola Basin) deposited in the Southern Hemisphere. Before and throughout the Iona-1 core OAE-2 interval, HSE abundance patterns indicate a mantle source for the unradiogenic Os, and are not consistent with an extraterrestrial impact trigger or contribution to LIP activity during OAE-2. This evidence for multiple high-flux pulses of LIP activity driving ocean deoxygenation has implications for the modern ocean, which is currently experiencing deoxygenation. These results provide new constraints on subsequent high-flux periods that extended the event. The first high-flux period started ∼60 Kyr after our selection of the onset of the CIE. The second and third high-flux periods started ∼270 and ∼400 Kyr after the onset of the CIE, respectively. After the third high-flux period, δ13Corg and Os isotope ratios shifted back to pre-excursion values over ∼585 kyr. In the Iona-1 core, OAE-2 lasted for 1.04 ± 0.12 Myr based on our selection of the CIE.

中文翻译：

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高分辨率锇数据记录了白垩纪海洋缺氧事件 2 (OAE-2) 期间岩浆活动的三个不同脉冲

摘要 海洋缺氧事件 2（OAE-2）发生在 Cenomanian-Turonian 边界（~94.1 Ma），是全球海洋化学和碳循环发生深刻变化的时期。该事件的特点是由大量有机碳埋藏、全球温室温度、海洋脱氧和大型火成岩省 (LIP) 活动驱动的海洋生物变化引起的碳同位素正偏移 (CIE)。整个显生宙的 LIPs 都有动态的岩浆通量，主要喷发的情节穿插着相对不那么强烈的喷发。整个显生宙 LIP 活动的一个可能触发因素已归因于外星撞击，因为在地质记录中同时发生了多个大型陨石坑、LIP 活动和大规模灭绝。在塞诺曼尼亚-图隆尼亚边界，在加拿大艾伯塔省西北部有一个直径为 25 公里（边缘到边缘）的复杂陨石坑，被称为 Steen 河撞击结构，其年代为 91 ± 7 Ma（Carrigy 和 Short，1968 年）。对于那些与 LIP 活动和大规模灭绝同时发现的陨石坑的另一种解释是，它们是由与 LIP 活动相关的大型爆炸事件造成的。与克拉通岩石圈下地幔柱孵化相关的爆炸事件已被认为会产生通常归因于撞击的地质特征（例如冲击石英、微球体等）。目前，OAE-2 期间 LIP 活动的触发因素，以及 LIP 活动的持续时间和喷发率的时间变化和幅度都没有得到很好的限制。为了解决 LIP 喷发率问题和 LIP 活动的触发因素，我们检查了锇 (Os) 丰度和同位素以及高度亲铁元素 (HSE；对于本研究：Re、Ru、Pd、Os、Ir、Pt) 丰度数据来自跨越 OAE-2 的连续沉积剖面。该剖面来自 Iona-1 核心中的 Eagle Ford Group，沉积在白垩纪西部内陆海道 (KWIS) 中。我们发现了三个高 Os 浓度区间，类似地幔的初始 Osi 同位素（初始 187Os/188Os）值为 ~0.16。这些间隔被解释为反映高通量 LIP 岩浆脉冲。在脉冲之间，观察到较低的 Os 丰度，具有约 0.20 的更多放射源 Osi 值，我们将其解释为高通量周期之间的低通量 LIP 活动。这种高通量和低通量 Os 浓度脉冲在高通量时期具有类似地幔的 Osi 值的趋势在沉积在 Iona-1 以北的另一个 KWIS 核心（USGS Portland #1）和深海钻探核心中发现项目 (DSDP) Site 530 孔 A（以下简称 DSDP 530A；在安哥拉盆地的纳米比亚近海钻探）沉积在南半球。在 Iona-1 核心 OAE-2 间隔之前和整个期间，HSE 丰度模式表明非放射性 Os 的地幔来源，并且与 OAE-2 期间外星撞击触发或对 LIP 活动的贡献不一致。LIP 活动的多个高通量脉冲驱动海洋脱氧的证据对目前正在经历脱氧的现代海洋具有影响。这些结果为随后延长事件的高通量时期提供了新的限制。在我们选择 CIE 的开始后，第一个高通量期开始于 ~60 Kyr。在 CIE 开始后，第二个和第三个高通量周期分别开始于 ~270 和 ~400 Kyr。在第三个高通量期之后，δ13Corg 和 Os 同位素比值在~585 kyr 范围内返回到偏移前的值。根据我们对 CIE 的选择，在 Iona-1 核心中，OAE-2 持续了 1.04 ± 0.12 Myr。