Vegetation response to southern California drought during the Medieval Climate Anomaly and early Little Ice Age (AD 800–1600)

Abstract

a b s t r a c tHigh-resolution studies of pollen in laminated sediments deposited in Santa Barbara Basin (SBB) core SPR0901-02KC reflect decadal-scale fluctuations in precipitation spanning the interval from AD 800 e1600. From AD 800e1090 during the Medieval Climate Anomaly (MCA) SBB sediments were dominated by xeric vegetation types (drought-resistant coastal sagebrush and chaparral) implying reduced precipitation in the southern California region. Drought-adapted vegetation abruptly decreased at AD 1090 and was rapidly replaced by mesic oak (Quercus) woodlands associated with an increased pollen flux into the basin. After a mesic interval lasting~100 years, pollen flux and the relative abundance of Quercus pollen dropped abruptly at AD 1200 when the rapid rise of chaparral suggests a significant drought similar to that of the MCA (~AD 800e1090). This brief resurgence of drought-adapted vegetation between AD 1200e1270 marked the end of the MCA droughts. A gradual increase in mesic vegetation followed, characterizing cool hydroclimates of the Little Ice Age (LIA) in coastal southern California.The presence of xeric vegetation in SBB coincides with major drought events recorded in tree rings and low lake levels elsewhere in California except for the brief drought between AD 1130e1160. Correlative diatom and terrigenous sediment input proxy records from SBB are largely supportive of the pollen record predominantly linking the MCA with drought and La Niña-like conditions and the LIA with wetter (more El Niño-like) conditions. Differences between paleoclimate proxies (pollen, diatoms, and terrigenous sediment) in SBB exist, however, possibly reflecting the temporal and spatial differences in the generation of each proxy record, as well as their individual sensitivity to climate change.

Pollen-based evidence of extreme drought during the last Glacial (32.6–9.0 ka) in coastal southern California

“…Rapid fluctuations in Quercus and other semi-arid lowland vegetation characterize the late Glacial to Holocene transition. The late-glacial resurgence of drought-adapted vegetation and low values of pollen flux resemble conditions not unlike those of the Glacial megadroughts in Lake Elsinore between~27.5 and 25.5 ka, and of Holocene multi-century droughts In SBB and at Zaca Lake during the Medieval Climate Anomaly (Heusser et al, 2014;Kirby et al, 2014).…”

“…Pollen assemblages deposited in LE, a pull apart basin, integrate pollen from vegetation of the San Jacinto drainage (~1240 km 2 ), including the broad (40 km), rolling hills of the San Jacinto Valley, as well as local runoff from the Elsinore Mountains. Pollen assemblages deposited in SBB, a deep, anoxic basin, integrate the composition and seasonal pollen production of vegetation in the large watershed of the Santa Clara River (4100 km 2 ) that drains the nearby Transverse Mountains and a narrow coastal plain (Heusser et al, 2014). Although average annual temperatures in LE and Santa Barbara (SB) are similar, LE lies~40 km inland on the leeward side of the Elsinore Mountains where average summer temperatures are~8 C higher than on the coast at SB.…”

“…Holocene pollen records from CSC are sparse and are generally low resolutiondcoarser than centennial scale (Cole and Liu, 1994;Heusser and Sirocko, 1997;Dingemans et al, 2014). An archival exception is a recent pollen reconstruction from SBB indicating a significant vegetative response to the drier-thanaverage Medieval Climate Anomaly (800e1300) and its transition into the early stages of the wetter-than-average Little Ice Age (~1300e1400) (Heusser et al, 2014).…”

The Holocene history of the North American Monsoon: ‘known knowns’ and ‘known unknowns’ in understanding its spatial and temporal complexity

“…Low lake levels are recorded in the Great Basin and California (Benson et al, 2002;Kirby et al, 2010), extending down as far as the TMVB. Tree ring records provide evidence of severe, centennial scale 'megadrought' across SW USA (Cook et al, 2004;Herweijer et al, 2007;Stahle et al, 2009) during the MCA, whilst drought-resistant taxa dominate pollen from laminated sediments in the Santa Barbara Basin between c. AD 800 and 1090 and again between c. AD 1200 and 1270 (Heusser et al, 2015). Megadroughts in the northern region have been associated with cooler SSTs in the tropical Pacific typical of La Niña type conditions (Kennett and Kennett, 2000;Cobb et al, 2003;Graham et al, 2007).…”

Millennial-to-centennial patterns and trends in the hydroclimate of North America over the past 2000 years

“…The Medieval period (800-1300 CE), in particular, included several decades-long droughts that have been widely analyzed to assess the underlying mechanisms (Coats et al, 2014;Cook et al, 2004;Herweijer et al, 2007;Meko, 2007;Routson et al, 2016a;Seager et al, 2007a), although earlier portions of the Common Era may have been at least as dry (Routson et al, 2011). Throughout this interval, the observed hydroclimate changes had important consequences for people, landscapes, and ecosystems (Heusser et al, 2015;Jones and 10 Schwitalla, 2008;Mason et al, 2004). As a result, the Common Era provides a useful baseline for considering future hydroclimatic changes and their impacts (Cook et al, 2015;Seager et al, 2007b).…”