NOAA/WDS Paleoclimatology - Lower Bear Lake, California 9170 Year Multiproxy Sediment Data

A well-dated, 9170 calendar year before present (cal yr BP) paleohydrologic reconstruction is presented from Lower Bear Lake in the San Bernardino Mountains of the coastal southwest United States. This new multi-proxy record is characterized by alternating organic-rich/carbonate-rich sediment units, interpreted to reflect hydrologically-forced changes in the lake's depositional environment. Our interpretation of the proxy data indicates nine decadal-to-multi-centennial pluvial episodes (PE) over the past 9170 cal yr BP. Of these nine inferred pluvials, five are interpreted as more pronounced based on their combined proxy interpretations: (PE-V) 9170?-8250, (PE-IV) 7000-6400, (PE-III) 3350-3000, (PE-II) 850-700, and (PE-I) 500-476 (top of core) cal yr BP. The Lower Bear Lake record indicates that the San Bernardino Mountains, source region for the Mojave River and its terminal playa, was wet during the same periods (within dating errors), to several of the major pluvials proposed from the lakes in the sink of the Mojave River. Our comparison extends north also to Tulare Lake, which drains the south-central western Sierra Nevada Mountains. This temporally and spatially coherent signal indicates that a similar climate forcing acted to increase regional wetness at various times during the past 9170 cal yr BP. As originally proposed by Enzel, Ely, and colleagues (e.g., Enzel et al., 1989; Enzel, 1992; Ely et al., 1994; Enzel and Wells, 1997), we too contend that Holocene pluvial episodes are associated with changing the frequency of large winter storms that track across a broad region at decadal-to-multicentennial timescales. We build upon their hypothesis through the addition of new and better-dated site comparisons, recent advances in the understanding of atmospheric rivers, and improved knowledge of the ocean-atmosphere dynamics that caused the early 20th century western United States pluvial.

本研究呈现了一项来自美国西南沿海圣贝纳迪诺山脉下贝尔湖的、定年精确的9170校准年前（cal yr BP）古水文重建序列。这套全新的多代用指标记录以富有机质/富碳酸盐沉积物单元交替发育为典型特征，其被解读为反映了湖泊沉积环境受水文作用驱动的变化。我们对代用指标数据的解读表明，在过去9170校准年前（cal yr BP）内，共存在9个十年至百年尺度的雨期事件（PE）。在这9个推断出的雨期中，基于综合代用指标解读，有5个被认为更为显著：分别为（PE-V）9170？-8250、（PE-IV）7000-6400、（PE-III）3350-3000、（PE-II）850-700以及（PE-I）500-476（岩芯顶部）校准年前（cal yr BP）。下贝尔湖的记录显示，作为莫哈韦河及其终端干盐湖的源区，圣贝纳迪诺山脉在莫哈韦河汇水区湖泊所提出的数个主要雨期时段内（定年误差范围内）均处于湿润状态。本研究还将对比范围向北扩展至流经内华达山脉西南中部的图莱里湖。这一时间与空间上均具有一致性的信号表明，在过去9170校准年前的不同时段，相似的气候强迫作用共同导致了区域湿度增加。正如恩泽尔、伊利及其团队最初提出的观点（例如Enzel等，1989；Enzel，1992；Ely等，1994；Enzel与Wells，1997），我们同样认为，全新世雨期事件与十年至百年尺度内大范围移动的大型冬季风暴的发生频率变化密切相关。我们通过新增定年更为精确的遗址对比数据、近年来对大气河流的研究进展，以及对引发20世纪早期美国西部雨期的海气动力学机制的深入认知，进一步完善了这一假说。