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South Coast Geological Society


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Meeting Date: Monday, January 12th
    - Social Hour Begins at 6:00PM

Location (Map Below): DoubleTree Club

Speaker: Dr. Matthew Kirby

Topic: 10,000 Years of Summer vs. Winter Influence on Mojave Hydroclimates where and what time?

Abstract:

Silver Lake is the modern terminal playa of the Mojave River. As a result, it is well located to record both influences from the winter precipitation dominated San Bernardino Mountains – the source of the Mojave River – as well as the late-summer to early-fall North American monsoon. Here, we present various physical and geochemical data from a new 8.2 m sediment core taken from Silver Lake, CA that spans modern through 14.8 kcal yrs BP. Age control is based on six bulk organic C radiocarbon dates processed with Bacon v2.2 to generate an age model. Texturally, the core varies between a clayey sand and a silty sand, often with abrupt sedimentological transitions. Our working hypothesis states that high percent clay values indicate persistent standing water wherein the deposition, accumulation, and preservation of fine grain sediment exceeds some undefined thickness that inhibits deflation during succeeding desiccation events or ephemeral lake environments. Based on this clay – lake status hypothesis, the sediment core is divided into five lake status intervals. Clay values are highest between 14.4 – 13.6 kcal yrs BP, coeval to Lake Mojave II. Clay values decrease abruptly at 13.6 kcal yrs BP (encapsulating the Younger Dryas) indicating a return to an ephemeral lake. At 11.6 kcal yrs BP, clay values rise abruptly indicating a return to a perennial lake; this early Holocene pluvial ended abruptly at 7.4 kcal yrs BP. From 7.4 – 4.2 kcal yrs BP, clay is low, but variable and mudcracks are common. At 4.2 kcal yrs BP, clay values increase but only moderately indicating a return to more frequent sustained perennial lakes. The early Holocene pluvial is likely a result of higher summer insolation, which generated a more intense and spatially expansive North American monsoon. Coupled with lower winter insolation and thus more winter storms across the region, Silver Lake flourished. A comparison to stable carbon isotope data from Leviathan Cave (NV), support our interpretation as indicated by more productive soils (i.e., wetter) (Lachniet et al., 2014). The resurgence of a wet Mojave ca. 4.2 kcal yrs BP is also supported by the NV cave data. We attribute this late Holocene pluvial to the strengthening of El Niño.

Speaker Information:

Matthew kirby obtained his A.B. from Hamilton College. He is an Associate Professor at Cal State Fullerton. He studies past climate using lake and wetland sediments.

Address: DoubleTree Club by Hilton 7 Hutton Drive, Santa Ana, CA

Meal: Regular or Vegetarian meals Complimentary tea and coffee will also be available

Cost: Members $30; Non-members $35; Students and Professors $15, $5 late fee for reservations made after 5pm the Friday prior to the meeting.


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