Anthropogenically enhanced chemical weathering and carbon evasion in the Yangtze Basin

Chemical weathering is a fundamental geochemical process regulating the atmosphere-land-ocean fluxes and earth’s climate. It is under natural conditions driven primarily by weak carbonic acid that originates from atmosphere CO2 or soil respiration. Chemical weathering is therefore assumed as positively coupled with its CO2 consumption in contemporary geochemistry. Strong acids (i.e. sulfuric- and nitric acid) from anthropogenic sources have been found to influence the weathering rate and CO2 consumption, but their integrated effects remain absent in the world largest river basins. By interpreting the water chemistry and overall proton budget in the Yangtze Basin, we found that anthropogenic acidification had enhanced the chemical weathering by 40% during the past three decades, leading to an increase of 30% in solute discharged to the ocean. Moreover, substitution of carbonic acid by strong acids increased inorganic carbon evasion, offsetting 30% of the CO2 consumption by carbonic weathering. Our assessments show that anthropogenic loadings of sulfuric and nitrogen compounds accelerate chemical weathering but lower its CO2 sequestration. These findings have significant relevance to improving our contemporary global biogeochemical budgets.


The Yangtze Basin
The Yangtze River (or Changjiang in Chinese) is the longest river in Asia, and one of the largest river systems in the world. It originates from the Qinghai-Tibet Plateau and flows 6 400 km eastward to the East China Sea. Its drainage area covers 1.8×10 6 km 2 , comprising about 1/5 of Chinese land area.
The Yangtze Basin is located in the subtropical monsoon climate zone, with an average annual precipitation of 1 100 mm. Spread throughout the basin are carbonaceous sedimentary minerals (limestone, sandstone and shale), comprising about half (44%) of the land area 2 . The chemistry in the Yangtze River is thus governed by carbonate rock weathering, with HCO 3 and Ca 2+ dominating the major ion composition 9,14 . Coal and ore deposits, rich in sulfides (e.g. pyrite), interbedded in the sedimentary rocks are rather common in the Yangtze Basin 9,14,21 .
The Yangtze Basin is the most important industrial region in China, generating 40% of the total national industrial production value. Agricultural land comprises 14% of the watershed, providing 40% of China's cereal production. Since the middle of last century, the basin has been subject to significant anthropogenic pollution, with acid rain being a regional environmental problem since the 1970s 9,41 .
Furthermore, widespread acid mining drainage has contributed significantly to the total acid loading through oxidation of sulfides to sulfuric acid in gangue and ore slag deposits 21 . Total loading of reactive nitrogen has also increased in the basin during the past 30 years, mainly due to nitrogen fertilization and increased emissions of NOx from fuel burning 42 . Concurrently, agricultural soils in the Yangtze Basin have become acidified due to overuse of nitrogen fertilizer 16 . Ammonium (NH 4 + ) has become a major pollutant in Yangtze River along with a rapid growth in urbanization along the river. Data from the Chinese national pollutant census state that 6.77×10 5 t NH 4 -N, mainly from domestic sewage, is discharged annually into the Yangtze River and its tributaries 18 .

DIC equilibrium calculation
Theoretical calculations of pH, dissolved CO 2 (H 2 CO 3 * ) and CO 2 partial pressure (P CO 2 ) were conducted according to CO 2 -H 2 O equilibriums (eq. S1-S4) in a closed system 43,44  and thus the partial pressure of CO 2 (P CO 2 ) in the closed system. Where this CO 2 supersaturated water is exposed to open air, the CO 2 will diffuse to atmosphere reducing the difference in P CO 2 . and HNO 3 ).
The nitrification of 6.77×10 5 t NH 4 + -N discharged to Yangtze River, with an annual water discharge of 9.51×10 11 m 3 , corresponds to an acidification of ca. 100 μmol L -1 H + , assuming that all of the NH 4 + is nitrified (eq. S7). Since ammonium nitrification is not the only proton source to Yangtze River, two additional acid loadings (50 and 200 μmol L -1 H + ) were assessed in order to cover the conceivable span in acid loading. The DIC range was studied from 500 to 2 500 μmol L -1 , spanning the current water chemistry of Yangtze River.

Proton loadings
It is assumed that all external proton loading is from carbonic acid (H + C ) and sulfuric acid (H + S ), or derived from nitrogen compounds (H + N ). Carbonic acid (H + C ) was estimated as the atmospheric CO 2 consumption flux by weathering, based on that aquatic CO 2 (H 2 CO 3 * ) deprotonates according to eq. S2 and S3 during chemical weathering. In this study the value used for CO 2 consumption in the Yangtze Basin was the average of compiled values found in literature, and assumed as constant for both past and present periods 2,3, 14,38 . All sulfate discharged to the sea at Datong station is presumed to have originally been sulfuric acid (H + S ) derived from acid deposition, sulfide oxidation and so on. This is disregarding a minor contribution from dissolution of evaporites (e.g. gypsum). Field surveys indicate that the contribution from evaporites to the sulfate flux is insignificant, though good quantitative data are lacking 2,9,14 . Furthermore, dissolution of sulfate from evaporites inherently leads to an equivalent flux of base cations, which is included in the measure for chemical weathering. This possible minor overestimation of acid loading due to evaporate dissolution will thus not influence the estimation of acidity budget.
Nitrogen processes have different contributions to the proton loading, with organic N mineralization, NH 4 + assimilation, NH 4 + nitrification and NO 3 assimilation generating 1 equivalent H + loss, 1 equivalent H + gain, 2 equivalent H + gains and 1 equivalent H + loss, respectively 16,45,46 . The overall acidification potential of nitrogen processes in the Yangtze Basin can be expressed by eq. S8.

Chemical weathering
Weathering of carbonate and silicate minerals consumes external protons and releases base cations through runoff. Total chemical weathering rates (expressed as their proton consumption) in the basin may therefore be deduced from the base cation discharge fluxes. However, the origin of base cations (i.e. carbonate or silicate mineral) need be distinguished in such deductions, since the equivalence ratios between base cation release and proton consumption are different between carbonate and silicate weathering (eq. S9, S11 and S12 conceptually consumes 1.0 equivalent H + (eq. S9). Carbonate weathering (W Carb ) is therefore estimated according to eq. S10, setting β to 4.8%. Minor contribution of (Ca 2+ +Mg 2+ ) flux by evaporate dissolution is not subtracted, thereby counterbalancing the erroneous contribution by evaporites to sulfuric acidity input, as stated above.
Weathering of silicate minerals release both monovalent (Na + +K + ) and divalent cations (Ca 2+ +Mg 2+ ). Monovalent base cation flux from silicate weathering ((Na + +K + ) Si ) consumes 1 equivalent H + (eq. S12). This loading is calculated from the riverine discharge after correction for the contribution from dissolution of evaporates and sea salt deposition, using chloride as tracer. Each divalent cation from silicate weathering ((Ca 2+ +Mg 2+ ) Si ) consumes 2 equivalent H + due to the formation of non-ionic ortho-silicic acid (eq. S11). The proton consumption by silicate mineral weathering (W Si ) is thus expressed by eq. S13.

Proton budget
It is assumed that all acid inputs are consumed through chemical weathering and alkalinity loss (-△Alk) within the basin. This is sound since the pH at Datong hydrological station remains stable and high through strong buffering by high bicarbonate concentrations. The acidity budget of the Yangtze Basin can thus be expressed by eq. S14. (eq. S14)

DIC budget calculation
The input-output budget of DIC is expressed by eq. S15. Weathering releases DIC to the Yangtze River through three mechanisms. Carbonic weathering of carbonate and silicate minerals captures the atmospheric CO 2 (Carbon Atm ) constituting the H + C flux expressed in eq. S14. Carbonic weathering of carbonate minerals (eq. S16) mobilizes an equivalent amount of atmospheric carbon and lithogenic paleocarbon (Paleocarbon Nat ) to the river. When carbonates instead are dissolved by anthropogenic strong acids (e.g. H 2 SO 4 and HNO 3 ), the paleocarbon released to river (Paleocarbon Ant ) is equal to the mole concentration of divalent cations (i.e. Ca 2+ and Mg 2+ ) (eq. S17, S18). Therefore, Paleocarbon Ant values were set as the difference between (Ca 2+ +Mg 2+ ) released from overall carbonate weathering (i.e. (Ca 2+ +Mg 2+ ) Carb in eq. S10) and from carbonic carbonate weathering. The latter value is equal to the (eq. S18) Discharge in eq. S15 denotes the DIC flux delivered to the sea, and was set equal to the efflux at Datong hydrological station. The difference between inputs and discharge is defined as the river processed DIC. This flux includes the CO 2 outgassing and fixation by aquatic primary producers through photosynthesis. Outgassing of riverine DIC occurs through two mechanisms: The major process is protonization of bicarbonate due to the proton loading and dehydration of carbonic acid, with a quantity that is equal to the reduction in inorganic carbon alkalinity (i.e. -△Alk in eq. S14). The minor one is from the oxidation of DOC. Global estimates show that about 70% of the DOC is oxidized to CO 2 in inland watercourses, estuaries and coastal regions [23][24][25][26][27] . The rest of the DOC is either precipitated in lakes, reservoirs, estuaries, or ocean and is defined as 'Assimilation' in eq. S15.

Terrestrial acidification and chemical weathering
Chemical weathering of silicate and carbonate minerals is the major process for consumption of external proton loadings to terrestrial systems. Its major products eventually are delivered to the ocean through inland watercourses. River chemistry is thus commonly used in contemporary geochemical studies as an important indicator of chemical weathering within watersheds 2,3 . Base cation concentrations are seen to increase significantly with sulfate and nitrate levels, except for nitrate in recent (2006) data ( Figure S2). The regression slopes for divalent cations (Ca 2+ +Mg 2+ ) are much steeper than those for monovalent cations (K + +Na + ), indicating considerably slower weathering rates of silicate minerals than for carbonates. This statistically derived result is consistent with more quantitative estimations found in literature 2,3,14 . We argue here that terrestrial acidification in the Yangtze Basin accelerates chemical weathering and thereby increases the flux of solutes to the watercourse. During recent years, nitrification of sewage ammonium has become an additional major source to nitrate in the Yangtze River 17,18 . This nitrification process releases proton directly to the river, without contribution to terrestrial chemical weathering. This is the cause for lack of any clear relationships between overall nitrate and base cation concentrations in the recent (2006) data ( Figure   S2c and S2d).
Contributions of carbonate weathering to river chemistry can also be evaluated using the strontium (Sr) concentration and its isotope ratio. River are found to be significantly positively and negatively correlated, respectively, with sulfate concentration ( Figure S3a and S3b). Nitric acid has similar effect on carbonate weathering as sulfuric acid 6,15 , though the in-river processing of ammonium to nitrate causes there to be no significant relations between nitrate and Sr geochemical data. Still, the terrestrial neutralization of sulfuric and nitric acids by especially carbonate weathering (eq. S17) increases the loading of bicarbonates to the Yangtze River. This causes the Sr concentration and 87 Sr/ 86 Sr ratio to also be correlated with DIC ( Figure S3c and S3d). In contrast to carbonic weathering (eq. S16), carbonate dissolution by strong acids (eq. S17, S18) releases bicarbonate derived only from sedimentary paleocarbon in the carbonate minerals.

Aquatic acidification and DIC outgassing
There are two major sources for external acid loading to the Yangtze River. In sub-catchments devoid of carbonates, residual acidity (mainly as Al 3+ and H + ) is leached from the soils and delivered to the upper streams. Some headwater streams in the Yangtze Basin are therefore acidic or even extremely acidic 19,41 . During recent years, ammonium discharged along with sewage has become a major cause for pollution in the Yangtze Rive 17 . Its nitrification (eq. S8) releases protons directly into the water, and is thus devoid of terrestrial acid neutralization. Instead this acid loading serves to acidify the Yangtze River.
At Zhutuo cross section, upstream of Chongqing, the water pH is generally found to decrease along with the increase in NH 4 + -N concentration over time during the past two decades (1991 to 2011) ( Figure S4a). A negative co-variation (r=0.16, n=251, p=0.01) between pH and ammonium is also observed at Jiujiang cross section, downstream of Jiujiang city, though there is no clear time trend ( Figure S4b). The lack of a clear pH decline with time at Jiujiang section is nevertheless not implying a lack of river acidification, because river acidification cannot be assessed simply by assessing temporary pH trends. Principle component analysis (PCA) was instead used to assess the underlying co-variations between H + , NH 4 + , dissolved oxygen (DO) and COD/BOD ( Figure S5). Figure S5a and S5b show inter-parameter relationship in regards to temporary co-variations in the monitoring data for the past two decades at Zhutuo and Jiujiang section, respectively. Ammonium (NH 4 + ) and H + show positive loading while DO was negatively loaded along the first component (PC1). This loading pattern indicates that NH 4 + increases the H + production while decreases (i.e. consuming) DO in river, which complies conceptually with the nitrification process (eq. S8). This PC1s explain 45.6 and 47.0% of the temporal variations in the measured parameters at Zhutuo and Jiujiang sites, respectively. Nitrification and resulting acid loading are therefore assessed to be the major components governing the water quality changes during past 21 years. Parameter loading plots from PCA of the regional water quality variations at the 19 key and 105 regular cross sections are shown in Figure S5c and S5d, respectively.
The loading plots a to d in Figure S5 appear generally similar. Acidification due to ammonium nitrification is therefore claimed to be the main governing mechanism for both temporal and spatial variations in these water quality parameters. In other words, the ammonium discharged with sewage is acidifying the Yangtze River.
Even though the river has been suffering from continuous acidification, substantial pH drops can not be expected in the Yangtze River. This is due to the fact that chemical weathering, especially carbonate weathering, delivers alkalinity to the river which efficiently is consuming the external acid loadings. Theoretical calculations demonstrate that nearly all of the total acid loading is consumed by protonation of dissolved carbonates, giving rise to a negligible proton (H + ) increase ( Figure S6). The results indicate therefore instead that the external acid input is mainly consumed by protonation of bicarbonate, enhancing CO 2 evasion as the system is open to the atmosphere. At any given DIC level, acid loadings elevate the CO 2 partial pressure (P CO 2 , along vertical axis of Figure S7), leading to enhanced CO 2 evasion. Furthermore, at any given acid loading P CO 2 increases with the DIC (along horizontal axis of Figure S7). This implies that the increase in DIC flux resulting from land acidification, as argued above, is augmenting the river outgassing of CO 2 . Within a conceivable range of acid (i.e. 50-200 μmol L -1 ) and DIC (500-2 500 μmol L -1 ) loading, there remains enough residual alkalinity to buffer river pH, keeping it stable. We therefore conclude that aqueous acidification in the Yangtze River accelerates the gas evasion of inorganic carbon, causing a significant release of CO 2 to the atmosphere, instead of causing a decline in pH. Geochim. Cosmochim. Acta 56, 2099-2111 (1992