Data_Sheet_1_Pacific Southwest United States Holocene Droughts and Pluvials Inferred From Sediment δ18O(calcite) and Grain Size Data (Lake Elsinore, California).PDF

Records of past climate can inform us on the natural range and mechanisms of climate change. In the arid Pacific southwestern United States (PSW), which includes southern California, there exist a variety of Holocene records that can be used to infer past winter conditions (moisture and/or temperature). Holocene records of summer climate, however, are rare from the PSW. In the future, climate changes due to anthropogenic forcing are expected to increase the severity of drought in the already water stressed PSW. Hot droughts are of considerable concern as summer temperatures rise. As a result, understanding how summer conditions changed in the past is critical to understanding future predictions under varied climate forcings. Here, we present a c. 10.9 kcal BP δ18O(calcite) record from Lake Elsinore, California, interpreted to reflect δ18O(lake water) values as controlled by over-water evaporation from summer-to-early fall. Our results reveal three millennial scale intervals: (1) the highly evaporative Early Holocene (10.55–6.65 kcal BP), (2) the less evaporative Mid-Holocene (6.65–2.65 kcal BP); and (3) the evaporative Late Holocene (2.65–0.55 kcal BP). These results are coupled with an inferred winter precipitation runoff (sand content) record from Kirby et al. (2010). Using these data together, we estimate the duration and severity of centennial-scale Holocene droughts and pluvials (e.g., high δ18O(calcite) values plus low sand content = drought and vice versa). Furthermore, the coupled δ18O(calcite) and sand data provide a generalized Holocene lake level history. The most severe, long-lasting droughts (i.e., maximum summer-to-early fall evaporation and minimum winter precipitation runoff) occur in the Early Holocene. Fewer, less severe, and shorter duration droughts occurred during the Mid-Holocene as pluvials became more common. Droughts return with less severity and duration in the Late Holocene. Notably, the Little Ice Age is characterized as the wettest period during the Late Holocene.

往昔气候记录可为我们揭示气候变迁的自然范围及其机制。在干旱的太平洋西南部美国（PSW）地区，包括南加州，存有丰富的全新世记录，可用于推断往昔冬季的气候条件（如湿度与/或温度）。然而，关于PSW地区夏季气候的全新世记录却相对罕见。未来，由于人为因素的影响，气候变迁预计将加剧PSW地区已有的水资源紧张状况，高温干旱尤为令人担忧。随着夏季温度的上升，了解夏季条件在过去的变迁对于理解在多样化气候驱动下的未来预测至关重要。在此，我们展示了来自加利福尼亚州伊尔西诺湖的约10.9千卡BP的δ18O（方解石）记录，该记录被解读为受夏季至初秋水面蒸发所控制的δ18O（湖水）值。我们的研究揭示了三个千年尺度的时段：（1）高度蒸发的早期全新世（10.55–6.65千卡BP），（2）蒸发较少的中全新世（6.65–2.65千卡BP）；以及（3）蒸发的晚期全新世（2.65–0.55千卡BP）。这些结果与Kirby等人（2010年）推断的冬季降水径流（沙含量）记录相结合。利用这些数据，我们估计了全新世尺度上百年级干旱和洪涝（例如，高δ18O（方解石）值加低沙含量代表干旱，反之亦然）的持续时间和严重程度。此外，耦合的δ18O（方解石）和沙数据提供了全新世湖面变化的历史概览。最为严重且持续时间最长的干旱（即夏季至初秋最大蒸发量和冬季最小降水径流）出现在早期全新世。中全新世期间，随着洪涝的增多，干旱事件减少、严重程度降低且持续时间缩短。晚期全新世中，干旱的严重程度和持续时间均有所降低。值得注意的是，小冰期是全新世晚期最为湿润的时期。