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Major shifts in hydroclimate have been documented during the last deglacial period and the Holocene in south‐central Alaska. Rare freshwater calcium carbonate (marl) deposits in lakes on the Kenai Peninsula can be used to reconstruct past changes in hydroclimate, including the influence of groundwater inflow to lakes. Here, the postglacial sediment sequence from groundwater‐fed Kelly Lake (60.514°N, 150.374°W) was analyzed for multiple proxies including isotopes of carbon and oxygen in marl calcite (δ13Cmarl and δ18Omarl), and isotopes of carbon (ẟ13COM) and abundances of C and N in organic matter. Bulk sediment analyses include organic matter and calcium carbonate (CaCO3) contents, visual stratigraphy and sediment flux. These analyses extend those of a previous paleoenvironmental reconstruction from Kelly Lake, which focused on sedimentary diatom oxygen isotopes and mass balance modeling over the past 10 000 years. Here, we show that Kelly Lake was deglaciated prior to 14.6 ka, and that by 14.0 ka marl dominated the sediments, with CaCO3 precipitation probably driven by groundwater input and mediated by shallow‐water charophytes. Marl accumulation decreased as organic and clastic inputs increased between ~12.2 and 11.5 ka. This shift, together with an increase in both δ13Cmarl and δ18Omarl values and a decrease in CaCO3 content, indicates an increase in the influence of meteoric water on the hydrologic budget under wet conditions, possibly driven by a strengthened Aleutian Low atmospheric pressure cell. A shift to lower δ13Cmarl and δ18Omarl values at ~11.5 ka is interpreted as an increase in the proportion of groundwater relative to meteoric water in the lake. Beginning around 9 ka, the proportion of meteoric water input continued to increase, the surrounding coniferous forest was established, and by 8 ka, CaCO3 accumulation ended. Our results elucidate the environmental conditions under which marl was deposited during the Lateglacial and early Holocene in this part of Alaska, and demonstrate how a variety of synoptic‐ and local‐scale climatic variables can converge to influence sedimentation in a groundwater‐fed lake. [ABSTRACT FROM AUTHOR]

Author(s): Christopher M. Moy (corresponding author) [1, 2]; Geoffrey O. Seltzer [1]; Donald T. Rodbell [3]; David M. Anderson [4] The variability of El Niño/Southern Oscillation (ENSO) during the Holocene [...]

The stratigraphy and chronology of loess deposits in the Mississippi Valley provide an important proxy record of the number and timing of glacial advances into the upper Mississippi drainage basin. The stratigraphy of loess and paleosols from two exposures in northwestern Tennessee reveals three major loess units that are correlated with the Loveland Loess, Roxana Silt, and Peoria Loess - from oldest to youngest. Detailed measurements of clay and secondary (pedogenic) iron contents suggest that the Loveland Loess may consist of multiple loess units which have been pedogenically welded to one another to form a composite Sangamon Soil. The mineralogy of the coarse silt fraction from one exposure reveals both primary mineralogical differences between the Peoria Loess and the Roxana Silt and pedogenic overprinting by the Sangamon Soil. Grain size and iron data suggest that the degree of pedogenic alteration increases from west to east, away from the loess source area, which probably reflects the role of sedimentation rate on the degree of pedogenic alteration of loess. However, only the Peoria Loess shows the expected exponential eastward decrease in thickness; the thickness of older loess units appears to be controlled more by the occurrence of localized erosion than by distance from source area. Most samples are resistant to solar resetting of the thermoluminescence signal, which, if unrecognized, could yield thermoluminescence ages that significantly underestimate the true age of the deposits. The best match between thermoluminescence age using the total bleach method and radiocarbon age of the Peoria Loess and Roxana Silt is achieved when either long durations of optical bleaching by sunlight ([greater than or equal to] 32 hr) or artificial ultraviolet (UV) light are applied to define the residual thermoluminescence level. Partial bleach analysis indicates that long-duration optical bleaching by sunlight or UV lamp bleaching does not systematically overbleach samples of Peoria Loess and Roxana Silt. Three thermoluminescence age estimates between 38 [+ or -] 4 ([+ or -]1[Sigma]) and 53 [+ or -] 5 ka from the basal half of the Roxana Silt, and six age estimates between 18 [+ or -] 2 and 25 [+ or -] 3 ka from the basal half of the Peoria Loess are consistent with numerous finite radiocarbon dates from around the central Mississippi Valley.

The degree of soil development on moraines and radiocarbon ages indicate that the Cordillera Oriental, a part of the eastern Andes in northern Peru (7 degrees 30'-7 degrees 55'S; 77 degrees 20'-77 degrees 35'W), was last glaciated during the last glacial maximum (marine isotope stage 2). This glaciation was the most extensive late Quaternary glaciation in the area. Radiocarbon-dated sedimentological changes in six sediment cores from three lakes and one bog reveal that the last deglaciation is marked by a two-stepped transition from rhythmically laminated inorganic sediment to massive, organic-rich sediment. The first step of this transition commenced before 12.1 + or - 0.2 ka (+ or - 1 Sigma) and probably by ca. 13.5 ka. Glaciers covered less than 50% of their last glacial maximum area by 12.1 + or - 0.6 ka, and cirques west of the main divide were probably ice-free by this time. A return to rhythmically laminated inorganic sediment in the lakes and the bog occurred between 12.1 + or - 0.6 and 10.3 + or - 0.4 ka. This may reflect an episode of increased stream discharge and glacier expansion during which glaciers reoccupied cirques above approximately 3,800 m. This glacier expansion adds to widespread evidence of a late-glacial readvance in the tropical Andes. All cirques have been ice-free since between 10.3 + or - 0.4 and 9.7 + or - 0.5 ka.

Debris flows have deposited inorganic laminae in an alpine lake that is 75 kilometers east of the Pacific Ocean, in Ecuador. These storm-induced events were dated by radiocarbon, and the age of laminae that are less than 200 years old matches the historic record of El Nino events. From about 15,000 to about 7000 calendar years before the present, the periodicity of clastic deposition is greater than or equal to 15 years; thereafter, there is a progressive increase in frequency to periodicities of 2 to 8.5 years. This is the modern El Nino periodicity, which was established about 5000 calendar years before the present. This may reflect the onset of a steeper zonal sea surface temperature gradient, which was driven by enhanced trade winds., The dramatic effects of the 1997-98 El Nino event have highlighted several shortcomings in our understanding of the El Nino-Southern Oscillation (ENSO) phenomenon (1). These shortcomings include the age of [...]