High-resolution environmental magnetic study of a Holocene sedimentary record from Zaca Lake, California

Abstract

Mineral magnetic studies of Holocene lake sediments recovered from Zaca Lake, in coastal southern California have yielded a 3000-year high-resolution record of terrestrial hydrologic variability and paleolimnology. Samples for magnetic analysis were obtained from an 8.73 m core recovered from the central region of Zaca Lake. Ages, constrained using a combined paleomagnetic and radiocarbon chronostratigraphy, yield sedimentation rates of 2–10 mm/yr with an average rate of 3 mm/yr over the 3000-year interval. Parameters reflecting decadal-scale variability in magnetic concentration (susceptibility, ARM, SIRM) and grain size (ARM/Chi) were measured every 2 cm. Additional rock magnetic tests on whole rock samples and magnetic separates indicate that magnetite and an iron sulfide are the chief magnetic carriers. Detailed analysis of the rock magnetic record reveals four major temporal intervals: 0–135 cm (~AD 2009–1757) the period of European settlement; 135–270 cm (~AD 1757–1275) the period of Native American, Chumash occupation of the watershed encompassing the bulk of the wet interval known as the little ice age (LIA); 270–600 cm (AD 1275–100) encompassing the drier Medieval Climate Anomaly (MCA); and 600–875 cm (2000–3000 ybp) an era dominated by a long interval of low magnetic mineral intensity and concentration that has been interpreted as a time of epic drought. Beginning with the cessation of drought conditions at Zaca Lake around ~2000 ybp, the magnetic proxy record of rock magnetic cyclicity and hydrologic variability shows a pronounced decadal to centennial-scale periodicity. We interpret these periodicities as oscillations between summer-dominated sedimentary sequences, characterized by whiting events with their resulting carbonate deposits, and winter dominated sedimentary sequences, typified by sulfide-rich clastics, reflecting lacustrine overturn and enhanced advection of detrital flux during winter storm events. The long-term variation of summer versus winter-dominated weather patterns reflects the climate variability of central California coast-ranges.

A comparison of late-Holocene hydrologic variability in California: Regionality of multi-decadal to centennial-scale droughts

“…There is also evidence for a trend in cycle duration (trend curves in Figure 7) toward even higher durations before 0 AD. This is consistent with evidence from multiple studies (Feakins et al, 2014; Kirby et al, 2014; Lund and Benson, 2018; Mensing et al, 2008, 2013; Platzman and Lund, 2019) for almost a millennial-scale drought in the USA Southwest from ~0 AD to 1000 BC. That interval would have duration of 500–1000 years.…”

“…The Mono Lake and Owens Lake results are consistent with the Pyramid Lake and Walker Lake data (Lund and Benson, 2018). Platzman and Lund (2019) carried out a similar paleoenvironmental study of late-Holocene (0-3000 cal. yrs.…”

“…Table 1 lists the mean age of each labeled wet and dry interval in the two records. Platzman and Lund (2019) studied two environmental proxies in Zaca Lake (California); clastic sedimentology (chi) and total inorganic carbonate (TIC) (Kirby et al, 2014). These proxies were measured with a stratigraphic sampling interval of ~5 cm, which led to a sample time intervals of ~8 years (Figure 2).…”

“…Several studies have also documented that the Sierra Nevada region lakes (Lund and Benson, 2018;Mensing et al, 2008Mensing et al, , 2013 and Zaca Lake (Feakins et al, 2014;Kirby et al, 2014;Platzman and Lund, 2019) have sustained a near millennial-scale drought between 0 AD and 1000 BC.…”

“…Platzman and Lund (2019) carried out a similar paleoenvironmental study of late-Holocene (0–3000 cal. yrs.…”