Sources and cycling of dissolved black carbon (DBC) in rivers and ocean revealed by carbon isotopes

<p><strong>Abstract</strong></p><p>Dissolve black carbon (DBC) has been recently recognized as an important fraction of dissolved organic carbon (DOC) in both rivers and ocean. It is estimated that about 10% of the riverine DOC transported by the world rivers could be DBC. The sources and fate of DBC in both rivers and ocean, however, is not well known. In this study, we present radiocarbon (<sup>14</sup>C) and stable carbon isotope (<sup>13</sup>C) measurements of DBC in several large rivers in China, and in coastal and open ocean waters. DBC was concentrated using solid phase extraction (SPE) method and quantified by chemothermal oxidation (CTO) method.</p><p>Concentrations of DBC varied in rivers depending on the drainage basin of the river and accounted for 3.7-7.6% of the riverine DOC pool. DBC was slightly lower, accounted for 2.9-5.9% of DOC in coastal and open oceans. Carbon isotope results indicate that DBC δ<sup>13</sup>C values were all slightly enriched (by 2-3‰) than the values of DOC in both rivers and ocean. The DBC Δ<sup>14</sup>C values varied largely in rivers and the values were significantly higher than DOC Δ<sup>14</sup>C values in rivers but similar to DOC Δ<sup>14</sup>C values in the ocean. Using a two-end member isotope mass balance model, we calculated that the most DBC (80%) with relatively young <sup>14</sup>C ages in the rivers was derived from biomass burning. Laboratory incubation studies also found that DBC released from recent charcoal was able to be utilized by bacteria, supporting the speculation that river transported DOC could be decomposed during estuaries mixing. Our study suggests that DBC is cycled in the same time scales with the DOC pool in the ocean and no extremely older DBC was identified as reported in other studies previously.</p>

APPLYING URANIUM-SERIES ISOTOPE GEOCHEMISTRY AND GEOCHRONOLOGY TO GREAT BASIN PLEISTOCENE PALEOHYDROLOGY

The widespread lake systems of the Basin and Range during the late Pleistocene indicate substantially greater moisture availability during glacial periods relative to modern. To determine the hydrography of the most recent lake cycle, we dated shoreline tufa deposits from wave-cut lake terraces in Surprise Valley, California. The lake hydrograph is constrained by 230Th-U ages on 22 tufa samples paired with 15 radiocarbon ages. This new lake hydrograph places the highest lake level 176 m above the present- day playa at >15.23 ± 0.36 ka cal BP (14C age). During the Last Glacial Maximum (LGM, ~19 to 26 ka), Lake Surprise stood at moderate levels, 65 to 99 m above modern playa. Temporally, the Lake Surprise highstand slightly postdates the Lake Lahontan highstand and corresponds to several post-LGM highstands and stillstands of smaller lake systems farther east. To further evaluate climatic forcings 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 2.5 to 18.2% (average = 9.5%) during the LGM relative to modern, compared to an approximately 75% increase in precipitation during the 15.23 ka highstand when lake surface area increased by 138%. LGM PMIP3 climate model simulations corroborate these findings, predicting an average precipitation increase of only 6.5% relative to modern, accompanied by a 28% decrease in total evaporation propelled by a 7°C decrease in mean annual temperature. LGM climate model simulations also suggest a seasonal decoupling of runoff and precipitation, with peak runoff shifting to the late spring. Based on our coupled analysis, we propose that moderate lake levels during the LGM were driven by reduced evaporation, a result of reduced summer insolation, and not by increased precipitation. Reduced evaporation primed Basin and Range lake systems, particularly smaller, isolated basins such as Surprise Valley, to rapidly respond to increased precipitation during late-Heinrich Stadial 1 (HS1, ~14.5 to 19 ka). Post-LGM highstands were potentially driven by increased rainfall during HS1 brought by latitudinally extensive and strengthened mid-latitude westerly storm tracks, the effects of which are recorded in the lacustrine and glacial records as far south as ~32°N. These results suggest that seasonal insolation, in particular the effect of summer insolation on lake evaporation, provides a previously under-investigated long-term driver of moisture availability in the western United States.

Source Apportionment of Particulate Matter Based on Carbon Isotope Mass Balance Model

In the process of source apportionment for particulate matter by mass balance model, the colinearity between various source profiles leads to the different analyzing results. For this reason, carbon isotopic apportionment based on the difference in carbon isotopic composition of particulate from different sources was put forward in this study. On the basis of chemical mass balance model, carbon isotope mass balance model is built to discriminate the sources including soil dust, coal dust and vehicle exhaust dust. This improved method has been used in the source apportionment of particulate in Taiyuan, and the results showed that the contributions of vehicle exhaust dust, coal dust and soil dust to air particulate in Taiyuan are 45%,13% and 18% respectively in heating season, while 23%,21% and16% in non-heating season. Therefore, the control of ash fly from burning coal is a long-term and arduous task.