Tropical shoreline ice in the late Cambrian: Implications for earth's climate between the Cambrian Explosion and the Great Ordovician Biodiversification Event
Tropical shoreline ice in the late Cambrian: Implications for earth's climate between the Cambrian Explosion and the Great Ordovician Biodiversification Event
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寒武纪晚期热带海岸线冰：寒武纪大爆发和奥陶纪大生物多样性事件之间对地球气候的影响
作者：
A. Runkel;T. Mackey;C. Cowan;D. Fox
寒武纪中晚期（Middle to Late Cambrian Time）513 ~ 488 Ma），生物多样性处于不稳定的高原期，沉积陆棚动物群遭受了反复的灭绝。这段鲜为人知的时间间隔将寒武纪大爆发与奥陶纪生物多样性大事件分开，通常被认为是持续温室条件的时间。我们提出的证据表明，在这个神秘的间隔截然不同的气候：功能指示流星冰保存完好，在晚寒武世赤道海滩存款，对应于一个货架灭绝事件。因此，寒武纪中晚期的地球至少是间歇性的寒冷，最好被认为是元古代特有的极端环境的一个无声的模拟，尽管这两个时期的冷却可能是响应于不同的触发因素。这种晚寒武纪的条件可能对奥陶纪辐射之前的演化产生了重大影响。引言了解古气候背景下发生的重大进化事件是最关键的，但最难以捉摸的，解释地球上生命的历史方面。寒武纪爆发和奥陶纪生物多样性大事件（GOBE）之间的时间约为2500万年。一个多样化程度较低的时期，被称为“晚寒武纪高原”（Bambach等人，2004）或“死亡间隔”（米勒等人，2006年）。中、晚寒武世海洋动物群多样性低，以三叶虫、磷质腕足动物和牙形石群落为主，但这些动物群的更替率很高（Bambach等人，2004;米勒，2004）。从钙质海底沉积物及其非均匀流变学性质的角度来看，这一时间间隔与元古代非常相似（Sepkoski，1982年;科万和James，1992年; Grotzinger和James，2000年; Knoll，2003年）。海底特征的相似性表明，至少有一些环境条件，代表了元古代，并存在于寒武纪辐射的后生动物，可能已经重新出现在中晚寒武纪的时间，之前GOBE。这种相似性可能不仅限于海底的性质。在本文中，我们描述了上寒武纪的特点，表明存在淡水冰在劳伦特赤道，这表明，像元古代，寒武纪的后半部分至少是全球性的间歇性寒冷。在北美南极洲内陆的Furongian（501.0-488.3 Ma）Jordan组的古海滩沉积物中，冰的异常内碎屑表明了这一点（图1）（伦克尔等人，2007年）。内碎屑保存的证据，脆-韧-脆的流变行为的变化，在他们的停留时间在古冲带的功能相同，在现代海滩砂在温带地区的冻-融-冻循环所造成的。芙蓉纪古赤道附近的冰冻气象条件的证据是在一个地层间隔，对应于全球灭绝的时间，长期以来一直被假定为由一个神秘的冷水海洋事件（帕尔默，1984年）。图1.位置图显示劳伦特晚寒武纪相带和相对于赤道（E）的位置以及劳伦特内陆的区域构造和地文特征。根据伦克尔et al.（2007）和Cocks and Torsvik（2002）的Laurentia古地理学修改。
Middle to late Cambrian time (ca. 513 to 488 Ma) is characterized by an unstable plateau in biodiversity, when depauperate shelf faunas suffered repeated extinctions. This poorly understood interval separates the Cambrian Explosion from the Great Ordovician Biodiversification Event and is generally regarded as a time of sustained greenhouse conditions. We present evidence that suggests a drastically different climate during this enigmatic interval: Features indicative of meteoric ice are well preserved in late Cambrian equatorial beach deposits that correspond to one of the shelf extinction events. Thus, the middle to late Cambrian Earth was at least episodically cold and might best be considered a muted analogue to the environmental extremes that characterized the Proterozoic, even though cooling in the two periods may have occurred in response to different triggers. Such later Cambrian conditions may have significantly impacted evolution preceding the Ordovician radiation. INTRODUCTION Understanding the paleoclimatic context within which major evolutionary events have occurred is one of the most critical, yet most elusive, aspects of interpreting the history of life on Earth. Between the Cambrian Explosion and the Great Ordovician Biodiversification Event (GOBE) is a ~25 m.y. period of subdued diversification, variously termed the “Late Cambrian Plateau” (Bambach et al., 2004) or the “Dead Interval” (Miller et al., 2006). Middle and late Cambrian marine fauna had low diversity, dominated by trilobite, phosphatic brachiopod, and conodont communities, yet these faunas experienced high turnover rates (Bambach et al., 2004; Miller, 2004). Viewed from the perspective of calcareous seafloor sediments and their non-uniformitarian rheological properties, this time interval is remarkable in its resemblance to the Proterozoic (Sepkoski, 1982; Cowan and James, 1992; Grotzinger and James, 2000; Knoll, 2003). The similarity in seafloor character suggests that at least some environmental conditions that typified the Proterozoic, and that existed prior to the Cambrian radiation of metazoa, might have reemerged in middle to late Cambrian time, preceding the GOBE. This resemblance may not be restricted to the nature of the seafloor. In this paper, we describe upper Cambrian features that indicate the presence of fresh-water ice at the Laurentian equator, suggesting that, like the Proterozoic, the latter half of the Cambrian was at least episodically globally cold. Ice is indicated by extraordinary intraclasts in ancient beach deposits of the Furongian (501.0–488.3 Ma) Jordan Formation in the cratonic interior of North America (Fig. 1) (Runkel et al., 2007). The intraclasts preserve evidence of brittle-ductile-brittle changes in rheological behavior during their residence time in the paleo-swash zone—features identical to those caused by freeze-thaw-freeze cycles in modern beach sands at temperate latitudes. This evidence for freezing meteoric conditions near the Furongian paleoequator is in a stratigraphic interval that corresponds in timing to a global extinction that has long been postulated to have been triggered by a cryptic cold-water oceanographic event (Palmer, 1984). Figure 1. Location maps showing Laurentian late Cambrian facies belts and position relative to the equator (E) and regional tectonic and physiographic features of the cratonic interior. Modified from Runkel et al. (2007) with paleogeography of Laurentia from Cocks and Torsvik (2002).