A Late Glacial–Holocene Tephrochronology for Glacial Lakes in Southern Ecuador
A Late Glacial–Holocene Tephrochronology for Glacial Lakes in Southern Ecuador
复制标题
厄瓜多尔南部冰川湖的晚冰期-全新世年代学
作者：
D. Rodbell;S. Bagnato;Jeffrey C. Nebolini;Geoffrey O. Seltzer;M. Abbott
尽管厄瓜多尔北部有许多活火山，但以前在该国南部三分之一的地区发现的远端火山很少。在本文中，我们在Las Cajas国家公园（2°40′-3°00′，79°00′-79°25′w）的五个冰川湖和一个沼泽的沉积物岩心中记录了薄（0.1 - 1.0 cm厚）远端tephras的存在，其中包括玻璃和/或角闪石和长石的斑晶。湖泊岩心包含5 ~ 7个tephras，每个tephras都具有诊断性的主要元素地球化学特征，这是通过对~ 710个玻璃碎片和~ 440个长石和角闪石斑晶的电子探针分析确定的。由于电子微束的作用，钠的损失导致Na含量降低了10±7 wt.%（±1σ），我们在对比沉积物岩心之间的碲之前对其进行了经验测定和修正。我们使用相似系数来关联沉积物岩心；对所有tephras的成对比较通常会在岩心之间产生明确的相关性。在所有或大部分岩心中可以追踪到6个岩心，而几个岩心只存在于一个或两个岩心中。沉积物岩心中保存的大型化石的26个加速器质谱14C测年为建立区域温度年代学提供了依据。广泛分布的tephras沉积在~ 9900、8800、7300、5300、2500和2200 cal yr B.P.，最古老的tephras沉积在~ 15500和15100 cal yr B.P.，但并非在所有岩心中都有发现。其中2个薄片可能与Volcán Cotopaxi侧翼的火山碎屑地层相关，1个薄片可能与Volcán Ninahuilca侧翼的地层相关；两座火山都位于厄瓜多尔中部。过去两千年来活火山的多次喷发导致沉积物岩心中没有碲，这意味着在湖泊中沉积了碲的早期火山喷发一定是特别大量的，或者在喷发时一定是盛行南风，或者两者兼而有之。
Abstract Despite the presence of numerous active volcanoes in the northern half of Ecuador, few, if any, distal tephras have been previously recognized in the southern one third of the country. In this article, we document the presence of thin (0.1–1.0-cm-thick) distal tephras comprising glass and/or phenocrysts of hornblende and feldspar in sediment cores from five glacial lakes and one bog in Las Cajas National Park (2°40′–3°00′S, 79°00′–79°25′W). The lake cores contain from 5 to 7 tephras, and each has a diagnostic major element geochemistry as determined from electron microprobe analysis of ∼710 glass shards and ∼440 phenocrysts of feldspar and hornblende. The loss of sodium with exposure to the electron microbeam causes a 10±7 wt.% (±1σ) reduction in Na content, which we empirically determined and corrected for before correlating tephras among the sediment cores. We use a similarity coefficient to correlate among the sediment cores; pair-wise comparison of all tephras generally yields an unambiguous correlation among the cores. Six tephras can be traced among all or most of the cores, and several tephras are present in only one or two of the cores. Twenty-six accelerator mass spectrometry 14C dates on macrofossils preserved in the sediment cores provide the basis for establishing a regional tephrochronology. The widespread tephras were deposited ∼9900, 8800, 7300, 5300, 2500, and 2200 cal yr B.P. The oldest tephras were deposited ∼15,500 and 15,100 cal yr B.P., but these are not found in all cores. Two of the tephras appear correlative with volcaniclastic strata on the flanks of Volcán Cotopaxi and one tephra may correlate with strata on the flanks of Volcán Ninahuilca; both volcanoes are in central Ecuador. The absence of tephras in sediment cores correlative with the numerous eruptions of active volcanoes of the past two millennia implies that the earlier eruptions, which did deposit tephras in the lakes, must have been either especially voluminous, or southerly winds must have prevailed at the time of the eruption, or both.