Long sediment cores recovered from the deep portions of Lake Titicaca are used to reconstruct the precipitation history of tropical South America for the past 25,000 years. Lake Titicaca was a deep, fresh, and continuously overflowing lake during the last glacial stage, from before 25,000 to 15,000 calibrated years before the present (cal yr B.P.), signifying that during the last glacial maximum (LGM), the Altiplano of Bolivia and Peru and much of the Amazon basin were wetter than today. The LGM in this part of the Andes is dated at 21,000 cal yr B.P., approximately coincident with the global LGM. Maximum aridity and lowest lake level occurred in the early and middle Holocene (8000 to 5500 cal yr B.P.) during a time of low summer insolation. Today, rising levels of Lake Titicaca and wet conditions in Amazonia are correlated with anomalously cold sea-surface temperatures in the northern equatorial Atlantic. Likewise, during the deglacial and Holocene periods, there were several millennial-scale wet phases on the Altiplano and in Amazonia that coincided with anomalously cold periods in the equatorial and high-latitude North Atlantic, such as the Younger Dryas.

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Glaciation in the humid tropical Andes is a sensitive indicator of mean annual temperature. Here, we present sedimentological data from lakes beyond the glacial limit in the tropical Andes indicating that deglaciation from the Last Glacial Maximum led substantial warming at high northern latitudes. Deglaciation from glacial maximum positions at Lake Titicaca, Peru/Bolivia (16°S), and Lake Junin, Peru (11°S), occurred 22,000 to 19,500 calendar years before the present, several thousand years before the Bølling-Allerød warming of the Northern Hemisphere and deglaciation of the Sierra Nevada, United States (36.5° to 38°N). The tropical Andes deglaciated while climatic conditions remained regionally wet, which reflects the dominant control of mean annual temperature on tropical glaciation.

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[1] Carbon isotopes are applied to estimate soil decomposition and physical mixing in well-drained forest soils by coupling new isotope and soil organic carbon (SOC) data with literature meta-analysis and carbon isotope mass balance modeling. New soil data results are presented for old-and second-growth forests in Southern Appalachia, USA and the Blue Mountains, Australia. The soils exhibit a SOC decrease and δ 13C increase with depth. The regressed gradient, termed β, of δ 13 C and the logarithm of SOC with depth in the soil column ranged from À1.09 to À1.65 for the measured soils. Twenty-four soils from 11 published studies across a range of cool temperate to tropical forest soils are used to show that β is dependent upon mean annual temperature (MAT) alone as well as mean annual temperature, mean annual precipitation, and soil texture, thus connecting the natural (nonlabeled) carbon isotope signature to the soil factors controlling soil decomposition and physical mixing. Carbon elemental and isotopic mass balance modeling of multiple SOC pools and multiple soil depths suggest that rates of decomposition and mixing are of the same order of magnitude for turnover in the studied forest soils. The results support the hypothesis that a pronounced negative, regressed β is indicative of isotopic fractionation during decomposition and physical mixing processes that occurs during soil turnover, and other hypotheses posed in the literature are marginalized using modeling and discussion. We discuss integration of the isotope method with existing SOC turnover models as a future research avenue.Citation: Acton, P., J. Fox, E. Campbell, H. Rowe, and M. Wilkinson (2013), Carbon isotopes for estimating soil decomposition and physical mixing in well-drained forest soils,

Abstract. Long-term secular variation in the isotopic composition of seawater fixed nitrogen (N) is poorly known. Here, we document variation in the N-isotopic composition of marine sediments (δ15Nsed) since 660 Ma (million years ago) in order to understand major changes in the marine N cycle through time and their relationship to first-order climate variation. During the Phanerozoic, greenhouse climate modes were characterized by low δ15Nsed (∼−2 to +2‰) and icehouse climate modes by high δ15Nsed (∼+4 to +8‰). Shifts toward higher δ15Nsed occurred rapidly during the early stages of icehouse modes, prior to the development of major continental glaciation, suggesting a potentially important role for the marine N cycle in long-term climate change. Reservoir box modeling of the marine N cycle demonstrates that secular variation in δ15Nsed was likely due to changes in the dominant locus of denitrification, with a shift in favor of sedimentary denitrification during greenhouse modes owing to higher eustatic (global sea-level) elevations and greater on-shelf burial of organic matter, and a shift in favor of water-column denitrification during icehouse modes owing to lower eustatic elevations, enhanced organic carbon sinking fluxes, and expanded oceanic oxygen-minimum zones. The results of this study provide new insights into operation of the marine N cycle, its relationship to the global carbon cycle, and its potential role in modulating climate change at multimillion-year timescales.

[1] A marine Permian-Triassic boundary (PTB) section at Nhi Tao, Vietnam, contains a series of at least 9 pyritic horizons characterized by concurrent decreases in pyrite S-(d 34 S py ) and carbonate C-isotopic compositions (d 13 C carb ). The first and largest of the events that precipitated these pyritic horizons was coincident with the Late Permian mass extinction, while subsequent events were generally smaller and occurred at quasiperiodic intervals of $20 to 30 ka. A near complete lack of organic carbon to drive bacterial sulfate reduction in sediment pore waters, among other considerations, argues against a diagenetic control for these relationships. Rather, the covariant patterns documented herein are most easily explained as the product of recurrent upwelling of anoxic deep-ocean waters containing 34 S-depleted hydrogen sulfide and 13 C-depleted dissolved inorganic carbon. The sulfide d 34S record of the study section represents a mixture of a small amount of isotopically heavy authigenic pyrite (formed via in situ bacterial sulfate reduction) with a generally larger quantity of isotopically light syngenetic pyrite precipitated within the water column during upwelling episodes. Although upwelling of toxic deepwaters has been invoked in earlier studies as a mechanism for the Late Permian marine mass extinction, this is the first study to (1) document patterns of pyrite-d 13 C carb covariation that strongly support upwelling as a major process at the PTB and (2) provide evidence of multiple, quasiperiodic upwelling events that may reflect reinvigoration of global-ocean overturn following a prolonged interval of Late Permian deep-ocean stagnation.Components: 5322 words, 5 figures.