Mark D. Shapley

A Coherent Signature of Anthropogenic Nitrogen Deposition to Remote Watersheds of the Northern Hemisphere

Deposition of reactive nitrogen from human activities occurred in the preindustrial era. Humans have more than doubled the amount of reactive nitrogen (Nr) added to the biosphere, yet most of what is known about its accumulation and ecological effects is derived from studies of heavily populated regions. Nitrogen (N) stable isotope ratios (15N:14N) in dated sediments from 25 remote Northern Hemisphere lakes show a coherent signal of an isotopically distinct source of N to ecosystems beginning in 1895 ± 10 years (±1 standard deviation). Initial shifts in N isotope composition recorded in lake sediments coincide with anthropogenic CO2 emissions but accelerate with widespread industrial Nr production during the past half century. Although current atmospheric Nr deposition rates in remote regions are relatively low, anthropogenic N has probably influenced watershed N budgets across the Northern Hemisphere for over a century.

Lateglacial and Holocene hydroclimate inferred from a groundwater flow-through lake, Northern Rocky Mountains, USA

Climate-driven variations in lake-groundwater exchange are recorded by sediments in groundwater-dominated lakes. A groundwater flow-through lake in west-central Montana (USA) registers latest Pleistocene and Holocene hydroclimatic variation in fluid and solute balance, as controlled by rates and timing of groundwater recharge. Early Holocene warming occurred under conditions of relative aridity and low groundwater throughflow, punctuated by a c. 450-yr episode of lake dilution centered on 11 000 cal. yr BP. Maximum evaporative concentration of lake waters, registered in both δ18O values and mineralogy of endogenic carbonates, coincided with the early-Holocene peak in insolation seasonality at about 9750 cal. yr BP. Subsequently, progressively decreasing lake residence time drove a sustained long-term decline in salinity while having a very subdued effect on mean δ18O values. We explain this decoupling by (1) limits placed on oxygen isotope sensitivity by groundwater throughflow, and (2) a shift toward greater summer rain contribution to lake inflow after mid-Holocene time. Superimposed multidecadal- and century-scale variation in lake—groundwater exchange generated high-frequency but low-amplitude isotopic oscillations throughout the record. High rates of groundwater throughflow maintaining low lake salinity similar to that observed today were established around 1400 cal. yr BP. We infer reduced regional stream baseflow, decrease in permanent wetlands (relative increase in ephemeral wetlands) and enhanced lake and wetland salinity prior to this time, relative to the late Holocene.

Negative correlations between Mg:Ca and total dissolved solids in lakes: False aridity signals and decoupling mechanism for paleohydrologic proxies

Lake-water Mg:Ca responds to endogenesis of carbonate minerals, providing a valuable indi- cator of paleosalinity when water-column cation ratios are preserved in calcareous lake sedi- ments. Typically Mg:Ca and total dissolved solids (TDS) correlate positively over a broad range of ionic compositions where calcium carbonate precipitation occurs. However, in groundwater- fed lakes where infl ow solutes are dominated by HCO 3 - , Ca 2+ , and Mg 2+ ions, and concentration of conservative solutes is limited by outfl ow, CaCO 3 formation and depletion of major source- water ions results in a negative correlation between Mg:Ca and TDS at low lake salinity. This relationship is promoted by high pCO 2 of infl owing groundwater, a common characteristic of groundwater-fed lakes such as our fi eld example, a groundwater fl ow-through lake in western Montana, United States. Equilibrium modeling of our fi eld example shows that evaporative evo- lution is expected to reverse the slope of the Mg:Ca/TDS relationship at moderately higher lake concentration. Generally, the TDS at the point of Mg:Ca/TDS reversal will depend on the initial concentration of less-reactive ions, and so on the source lithology for groundwater solutes.

Lake levels in a discontinuous permafrost landscape: Late Holocene variations inferred from sediment oxygen isotopes, Yukon Flats, Alaska

ABSTRACT During recent decades, lake levels in the Yukon Flats region of interior Alaska have fluctuated dramatically. However, prior to recorded observations, no data are available to indicate if similar or more extreme variations occurred during past centuries and millennia. This study explores the history of Yukon Flats lake origins and lake levels for the past approximately 5,500 years from sediment analyses guided by previous work on permafrost extent, thermokarst, and modern isotope hydrology. Sediments dated by 210Pb and AMS radiocarbon indicate stable chronologies following initial lake initiation. Subsequent lithology is autochthonous, and oxygen isotope ratios of endogenic carbonate reflect lake level change at multiple time scales. Sediment results indicate high lake levels between approximately 4000 and 1850 cal yr BP, which is interpreted to reflect wetter-than-modern conditions. Lower lake levels with short-lived high stands during the past approximately 800 years reflect generally arid conditions with brief wet intervals similar to the region’s moisture regime today. The millennial trend is one of increasing aridity and corresponds closely with fire reconstructions and regional paleoclimatic trends. We conclude that high-magnitude lake-level fluctuations and decadal scale trends occurred before the observational period and are persistent hydroclimatic features of the Yukon Flats region.