Daniel E. Ibarra

Rise and Fall of Late Pleistocene Pluvial Lakes in Response to Reduced Evaporation and Precipitation: Evidence from Lake Surprise, California

Widespread late Pleistocene lake systems of the Basin and Range Province indicate substantially greater moisture availability during glacial periods relative to modern times, but the climatic factors that drive changes in lake levels are poorly constrained. To better constrain these climatic forcing factors, we present a new lacustrine paleoclimate record and precipitation estimates for Lake Surprise, a closed basin lake in northeastern California. We combine a detailed analysis of lake hydrography and constitutive relationships describing the water balance to determine the influence of precipitation, evaporation, temperature, and seasonal insolation on past lake levels. At its maximum extent, during the last deglaciation, Lake Surprise covered 1366 km 2 (36%) of the terminally draining Surprise Valley watershed. Using paired radiocarbon and 230 Th-U analyses, we dated shoreline tufa deposits from wave-cut lake terraces in Surprise Valley, California, to determine the hydrography of the most recent lake cycle. This new lake hydrograph places the highest lake level 176 m above the present-day playa at 15.19 ± 0.18 calibrated ka ( 14 C age). This significantly postdates the Last Glacial Maximum (LGM), when Lake Surprise stood at only moderate levels, 65–99 m above modern playa, similar to nearby Lake Lahontan. To evaluate the climatic factors associated with lake-level changes, we use an oxygen isotope mass balance model combined with an analysis of predictions from the Paleoclimate Model Intercomparison Project 3 (PMIP3) climate model ensemble. Our isotope mass balance model predicts minimal precipitation increases of only 2%–18% during the LGM relative to modern, compared to an ∼75% increase in precipitation during the 15.19 ka highstand. LGM PMIP3 climate model simulations corroborate these findings, simulating an average precipitation increase of only 6.5% relative to modern, accompanied by a 28% decrease in total evaporation driven by a 7 °C decrease in mean annual temperature. LGM PMIP3 climate model simulations also suggest a seasonal decoupling of runoff and precipitation, with peak runoff shifting to the late spring–early summer from the late winter–early spring. Our coupled analyses suggest that moderate lake levels during the LGM were a result of reduced evaporation driven by reduced summer insolation and temperatures, not by increased precipitation. Reduced evaporation primed Basin and Range lake systems, particularly smaller, isolated basins such as Surprise Valley, to respond rapidly to increased precipitation during late-Heinrich Stadial 1 (HS1). Post-LGM highstands were potentially driven by increased rainfall during HS1 brought by latitudinally extensive and strengthened midlatitude westerly storm tracks, the effects of which are recorded in the region9s lacustrine and glacial records. These results suggest that seasonal insolation and reduced temperatures have been underinvestigated as long-term drivers of moisture availability in the western United States.

Uranium isotopes in soils as a proxy for past infiltration and precipitation across the western United States

The intermittent presence of large Pleistocene lakes in the southwestern interior of North America, a region that is now a semi-arid desert, suggests repeated oscillations between profoundly different climatic conditions. The origin of these shifts is still unresolved due to inconsistencies in existing climate proxy data (for example, pollen, lake levels, and oxygen isotopes in speleothems). To resolve the inconsistencies in the water balance over the last 10 to 60 kyr, we use uranium isotopic variations in secondary soil minerals to quantify net infiltration and precipitation along a north-south transect in western North America. We show that winter infiltration increased by 30 to 100 percent, and precipitation by a lesser amount, in the valleys of the Great Basin and Mojave deserts between 60 and ∼26 ka. This increase in infiltration and precipitation preceded the Last Glacial Maximum (LGM) and the timing of most lake highstands in the region by 5 to 10 kyr, respectively, suggesting a possible Last Precipitation Maximum (LPM) that coincided with a minimum in winter insolation. Subsequent decreases in infiltration and precipitation after the LGM can be reconciled with the timing of lake highstands if colder summer temperatures due to a minimum in summer insolation reduced lake evaporation. The soil records, combined with a range of proxy data, suggest that seasonal insolation is the long-term driver for large shifts in both precipitation and surface water variability in the region.

A hot and high Eocene Sierra Nevada

Despite broad interest in determining the topographic and climatic histories of mountain ranges, the evolution of California’s Sierra Nevada remains actively debated. Prior stable isotope–based studies of the Sierra Nevada have relied primarily on hydrogen isotopes in kaolinite, hydrated volcanic glass, and leaf n -alkanes. Here, we reconstruct the temperature and elevation of the early Eocene Sierra Nevada using the oxygen isotope composition of kaolinitized granite clasts from the ancestral Yuba and American Rivers that drained the windward (Pacific) flank of the Sierra Nevada. First, we evaluated the possible contributions of hydrogen isotope exchange in kaolinite by direct comparison with oxygen isotope measurements. Next, we utilized differences in the hydrogen and oxygen isotope fractionation in kaolinite to constrain early Eocene midlatitude weathering temperatures. Oxygen isotope geochemistry of in situ kaolinites indicates upstream (eastward) depletion of 18O in the northern Sierra Nevada. The δ18O values, ranging from 11.4‰ to 14.4‰ at the easternmost localities, correspond to paleoelevations as high as 2400 m when simulating the orographic precipitation of moisture from a Pacific source using Eocene boundary conditions. This result is consistent with prior hydrogen isotope studies of the northern Sierra, but oxygen isotope–based paleoelevation estimates are systematically ~500–1000 m higher than those from hydrogen-based estimates from the same samples. Kaolinite geothermometry from 16 samples produced early Eocene weathering temperatures of 13.0–36.7 °C, with an average of 23.2 ± 6.4 °C (1σ). These kaolinite temperature reconstructions are in general agreement with paleofloral and geochemical constraints from Eocene California localities and climate model simulations. Our results confirm prior hydrogen isotope–based paleoelevation estimates and further substantiate the existence of a hot and high Eocene Sierra Nevada.

The Neogene de-greening of Central Asia

There remains substantial debate concerning the relative roles of tectonics and global climate in driving the evolution of climate in Central Asia. Today, interior Asia—including the Taklamakan, Gobi, and Ordos Deserts—is exceptionally arid and is surrounded by distinct rainfall boundaries, such as those generated by the Asian monsoon systems to the east and south and those generated by high topography to the north and west. Determining how and why these boundaries have varied over the Neogene is hindered by the lack of a single proxy that can be broadly applied through space and time. We construct isoscapes of pedogenic carbonate δ 13 C (δ 13 C c ) over the Neogene in Asia by combining a compilation of 2236 published measurements with new data from three localities in northern Central Asia. Pedogenic carbonate δ 13 C records local aridity—excepting localities impacted by C 4 grasslands and during large changes in atmospheric p CO 2 —through variations in soil respiration, depth of carbonate formation, and the effect of water stress on plant δ 13 C. Together, these effects reflect changes in both primary productivity and mean annual precipitation. Throughout the Neogene, we find consistent and exceptionally high δ 13 C c in interior Asia with a ring of lower δ 13 C c that demarcates higher precipitation. This persistent ring of lower δ 13 C c corresponds in the south and east with the influence of the Asian monsoon systems; in the west and north, it reflects both orographic rainfall due to uplift of the Tian Shan and to moisture delivery by the mid-latitude westerlies. Finally, δ 13 C c and, hence, aridity increases regionally in the latest Neogene, reflecting the effects of Northern Hemisphere glaciation and cooling. This widespread “de-greening” would have increased regional albedo and modified basin-scale water balances, resulting in greater dust fluxes due to reduced vegetative cover and precipitation.

Critical zone structure controls concentration-discharge relationships and solute generation in forested tropical montane watersheds

Concentration-discharge (C-Q) relationships are poorly known for tropical watersheds, even though the tropics contribute a disproportionate amount of solutes to the global ocean. The Luquillo Mountains in Puerto Rico offer an ideal environment to examine C-Q relationships across a heterogeneous tropical landscape. We use 10-30 years of weekly stream chemistry data across ten watersheds to examine C-Q relationships for weathering products (SiO2(aq), Ca2+, Mg2+, Na+) and biologically-controlled solutes (dissolved organic carbon [DOC], dissolved organic nitrogen [DON], NH4+, NO3-, PO43-, K+, SO42-). We analyze C-Q relationships using power-law equations and a solute production model, and use Principal Component Analysis to test hypotheses regarding how the structure of the Critical Zone controls solute generation. Volcaniclastic watersheds had higher concentrations of weathering solutes and smaller tributaries were approximately 3-fold more efficient at generating these solutes than larger rivers. Lithology and vegetation explained a significant amount of variation in the theoretical maximum concentrations of weathering solutes (r2 = 0.43 - 0.48) and in the C-Q relationships of PO43- (r2 = 0.63) and SiO2(aq) (r2 = 0.47). However, the direction and magnitude of these relationships varied. Across watersheds various forms of N and P displayed variable C-Q relationships, while DOC was consistently enriched with increasing discharge. Results suggest that PO43- may be a useful indicator of watershed function. Relationships between C-Q and landscape characteristics indicate the extent to which the structure and function of the critical zone controls watershed solute fluxes.

Concentration–discharge patterns of weathering products from global rivers

Quantifying the functional relationships relating river discharge and weathering products places key constraints on the negative feedback between the silicate weathering and climate. In this study we analyze the concentration–discharge relationships of weathering products from global rivers using previously compiled time-series datasets for concentrations and discharge from global rivers. To analyze the nature of the covariation between specific discharge and concentrations, we use both a power law equation and a recently developed solute production equation. The solute production equation allows us to quantify weathering efficiency, or the resistance to dilution at high runoff, via the Damköhler coefficient. These results are also compared to those derived using average concentration–discharge pairs. Both the power law exponent and the Damköhler coefficient increase and asymptote as catchments exhibit increasingly chemostatic behavior, resulting in an inverse relationship between the two parameters. We also show that using the distribution of average concentration–discharge pairs from global rivers, rather than fitting concentration–discharge relationships for each individual river, underestimates global median weathering efficiency by up to a factor of ~10. This study demonstrates the utility of long time-series sampling of global rivers to elucidate controlling processes needed to quantify patterns in global silicate weathering rates.

DIAGENETIC AND PALEOENVIRONMENTAL CONTROLS ON LATE CRETACEOUS CLAY MINERALS IN THE SONGLIAO BASIN, NORTHEAST CHINA

Sedimentary and diagenetic processes control the distribution of clay minerals in sedimentary basins, although these processes have seldom been studied continuously in continental sedimentary basins. The Songliao Basin, northeast China, is a large continental, petroleum-bearing basin, and provides a unique study site to understand the sedimentary and diagenetic processes that influence clay assemblages. In this paper, the clay mineralogy of a 2500 m-thick Late Cretaceous (late Turonian to Maastrichtian) terrestrial sedimentary succession (SK-1s and SK-1n cores), retrieved by the International Continental Scientific Drilling Program in the Songliao Basin, was examined. The objective was to determine the diagenetic and paleoenvironmental variations that controlled the formation of clay mineral assemblages, and to determine the thermal and paleoenvironmental evolution of the basin. The results from both cores show that illite is ubiquitous through the succession, smectite is frequently encountered in the upper strata, and ordered mixed-layer illite-smectite (I-S), chlorite, and kaolinite are abundant in the lower strata. Burial diagenesis is the primary control on the observed decrease of smectite and increasing illite, I-S, and chlorite with depth. Observations of clay-mineral diagenesis are used to reconstruct the paleotemperatures and maximum burial depths to which the sediments were subjected. The lowermost sediments could have reached a maximum burial of ~1000 m deeper than today and temperatures ~50°C higher than today in the latest Cretaceous. The transition of smectite to I-S in the SK-1 cores and the inferred paleotemperatures provide new constraints for basin modeling of oil maturation at elevated temperatures in the Songliao Basin. Authigenic kaolinite and smectite are enriched in sandstones with respect to the coeval mudstones from the SK-1n core, as a result of early diagenesis with the participation of primary aluminosilicates and pore fluids. In the upper part of both SK-1 cores, variations in smectite and illite were controlled primarily by paleoenvironmental changes. Increases in smectite and decreases in illite from the late Campanian to Maastrichtian are interpreted as resulting from increasing humidity, a conclusion consistent with previous paleoenvironmental interpretations.

Quantifying closed-basin lake temperature and hydrology by inversion of oxygen isotope and trace element paleoclimate records

Lake systems are important paleoclimate archives that preserve ecosystem and hydrologic responses to critical periods in Earth history, such as carbon cycle perturbations and glacial-interglacial cycles. Geochemical measurements of biogenic carbonate (for example, δ18O, δ13C, 87Sr/86Sr, [Li], [U], [Sr], and [Mg]) are indicators of hydrologic variability in lake systems throughout the geologic record. In this study, we present a new closed-basin lake modeling approach, HyBIM (the Hydrologic Balance Inverse Model) that employs a system of total differential equations and uses the measured δ18O, Sr/Ca, and Mg/Ca of biogenic carbonate to determine changes in temperature, runoff, and lake evaporation. Using equally-spaced time steps, these equations are simultaneously solved to constrain the hydrologic parameters of the lake as recorded in biogenic carbonate. We use a Monte Carlo approach to account for uncertainty in the input parameters, such as δ18O temperature relationships, partition coefficient uncertainty, and watershed solute chemistry. For illustrative purposes, we apply the model to two ostracod valve datasets covering different timescales: (1) the Cretaceous Songliao Basin, northeast China, and (2) Holocene Lake Miragoane, Haiti. Modern water measurements of water isotopes and cation concentrations from each location are required as model inputs. We compare our modeling results with author interpretations and geologic observations. The modeling approach presented in this study can be applied to other closed-basin lake records, can be modified for other calcifying species (for example, gastropods or mollusks) or with calibration to inorganic lacustrine carbonate. In addition, this approach holds promise for extension with additional proxy measurements (that is, δD, U/Ca or Li/Ca) and changing source area on tectonic timescales using proxies that reflect changing source lithology (that is, Sr and Pb isotopes). Future incorporation of age model uncertainty in the Monte Carlo approach will also provide utility by quantifying temporal uncertainty on the hydrologic response recorded by lake sediments.