Carbon Burial in Young Tropical Reservoirs Is Higher at Lower Latitudes *

Man-made environments such as tropical hydroelectric reservoirs alter the preexisting carbon (C) cycle and remove C from circulation through burial in sediments. Carbon burial (CB) was measured using the silica-tracer method during four field surveys in the less than six-year-old Belo Monte tropical reservoir. Fresh C sedimentation was also measured. Belo Monte’s CB median rate 276 (n = 84; min 0; max 352,625 mg C∙m −2 ∙d −1 ) is within the range (230 to 436 mg C∙m −2 ∙d −1 ) of CB rates measured further downstream at the Xingu Ria and higher than the averaged over 50 years oceanic rate 244 mg C∙m −2 ∙d −1 estimated for an increasingly deoxygenated ocean. Carbon burial median rates of tropical reservoirs with similar age and trophic state correlate inversely with latitude at a rate of 17.5 mg C∙m −2 ∙d −1 per degree. Carbon burial efficiency of these reservoirs correlates positively with latitude at a ratio of 0.22% per degree.

tion, agriculture, resource extraction, overpopulation and flooding. Science and technology contribute with assessments and attempts to prevent and alleviate the state of affairs, such as Awuh (2021)'s identification of adaptation measures employed to combat urban-heat-island effects, Salameh (2021)'s investigation on how a whole groundwater stock can be exhausted by exploitation of deep groundwater resources and Liu et al. (2021)'s proposal for long-term implementation of the sustainable supply chain method to ameliorate the impacts of water diversion projects.
Landscape change comes with altered carbon (C) circulation, such as the originated by hydroelectric reservoir creation (Kopittke et al., 2021;Reynolds, 2021). In these reservoirs autochthonous organic matter is produced (Kunz et al., 2011), carbon burial (CB) can sustain methane emission  and unfavorable decomposition conditions promote CB (Isidorova et al., 2019). Sedimentation rates are controlled by precipitation, water inflow, water residence time and surrounding reservoir land use (Leite, 1998). Quantification of the sediment magnitude and its increase (Lewis et al., 2013;Miranda & Mauad, 2014;Hilgert & Fuchs, 2015) and its C concentration can and have been used to determine sedimentary C stock increase (Bernardo et al., 2017). Tropical reservoirs emit more methane (Sikar et al., 2005;Bertassoli et al., 2021) and bury 3 times more carbon (Sikar et al., 2009) compared to the pre-flooded area.
Despite the ongoing debate about incorporation, or not, of C that is buried by hydroelectric reservoirs, into greenhouse gas inventories (IPCC, 2019) there is increasing action to acknowledge (Mendonça et al., 2012) and quantify C burial rates by these reservoirs (Teodoru et al., 2012;Wang et al., 2019;Phyoe et al., 2020).
With increasing attention to quantification of anthropogenic C emissions, it becomes also more imperative to assess the expanding realm of man-caused C retention rates. In this respect, Hamido et al. (2016) measured significant C storage (252 -638 mg C•m −2 •d −1 ) in domestic turfgrass lawns in Alabama USA and Dilla et al. (2019) conclude that by increasing the density of f. albida trees from 5.80 to 100 ha −1 in a tropical parkland (8.5˚N) soil C sequestration could be estimated as 132 mg C•m −2 •d −1 for 42 years.
The C sink status of the sediments in man-made reservoirs has long been foreseen by Mulholland & Elwood (1982)

Materials and Methods
Three definitions relevant to this work are: 1) OC sedimentation rate is the daily quantity of total (aka "fresh") OC that lands on the sediment. Some of the fresh OC will undergo decomposition and return to circulation while another portion will escape decomposition and remain permanently sedimented.
2) OC burial rate (CB) is the daily amount of OC that escaped decomposition and therefore is out of the carbon circulation process and is permanently sedimented.
3) Carbon burial efficiency (CBE) is the ratio "organic carbon burial rate/ organic carbon sedimentation rate".

E. Sikar et al. Journal of Geoscience and Environment Protection
Silica was used as an OC burial tracer (Sikar et al., 2012). Concomitantly and for the sake of comparison with OC burial rates, fresh OC sedimentation rates were also measured (Sikar et al., 2012).
Sediment dredging and sediment trap deployment procedures were performed at each of the 24 sampled sites (Figure 3).

Results and Discussion
Water median depths were smaller during the second survey and higher during the third, for all three sampled environments (Table S1).
Within Aug 2019-Jun 2021 time span, organic carbon burial rates ( Figure 4 and Possibly due to reservoir youngness, a somewhat generalized paucity of dredgable sediment and even more so of sediment with "expected" appearance (muddy, clayey, layered) was noted. We assumed that with flooding and subsequent se-   (terrestrial particulate organic carbon) rather than autochthonous organic matter burial in sediments of inland waters and also, vegetal remains have been observed in the 111 -88 and 48 -11.5 cm deep sediment layers of an Amazonia floodplain lake by Moreira-Turcq et al. (2004). However, anthropogenic allochthonous carbon in more severely impacted waterbodies, such as the semi-treated sewage flowed into subtropical eutrophic Lake Donghu located in Wuhan City/ China, might not be as recalcitrant as allochthonous carbon of natural origin (Yang et al., 2008).
Measured CB rates of all four surveys varied between 0 and 352,625 mg C•m −2 •d −1 (Figure 4 and Table S1). The lower (null) rate is due to the amount of carbon in sediment sample being below the detection limit of the analytical balance. The higher rate is because of the high (54%) C content in what appeared to be a preserved seed in the sediment sample and the high (10,918 mg m −2 •d −1 ) sedimentation rate of silica. BM Reservoir's CB median rate 276 mg C•m −2 •d −1 is higher than those found in tropical reservoirs Serra da Mesa (14˚S; median 87 mg C•m −2 •d −1 ; n = 14; min 19; max 516) and Manso (15˚S; 62; 9; 18; 212) measured when they were between 3.7 and 6.7 years old (Sikar et al., 2012).
This reveals a robust (R 2 = 0.99) inverse correlation ratio of 17.5 mg C•m −2 •d −1 per degree South, for young tropical hydroelectric meso-oligotrophic reservoirs located between 3˚S and 15˚S in Brazil ( Figure 6). If the burial efficiency increase rate of 0.22% yr −1 measured in tropical reservoirs almost two decades ago (Sikar et al., 2012) holds it can be used with the inverse correlation ratio 17.5 mg C•m −2 •d −1 per degree South here obtained to predict burial rates in tropical reservoirs of similar characteristics e.g. flooded land type and trophic state. Curuá-Una is a hydroelectric oligotrophic reservoir inaugurated in 1977 (44 years old) located 266 km NW of BM and only 0.5˚ north. Assuming it buried a "corrected for latitude" 276 mg C•m −2 •d −1 when it was 5 years old (as BM) an estimate In comparison, lifetime average carbon burial rates measured in Curuá-Una 4 years ago using a linear model of sediment accumulation rate and organic carbon accumulation rate yielded a 20% smaller rate (249 mg C•m −2 •d −1 (Quadra et al., 2020) than what we estimated for this present year 2021 (Equation (1)).
The CB/latitude ratio here noted will not hold as trophic states increase. In extreme cases sediment dredging is necessary in order to restore volume capacity. This was observed in the subtropical urban stretches (23.5˚S -23.6˚S) of Brazilian rivers Tietê and Pinheiros both located in the megacity of São Paulo and both with high emissions of methane and carbon dioxide, >5% nitrogen concentration in bubbles (Sikar et al., 2019) and high concentrations of ammonium (>15 mg + 4 N-NH L −1 ; (Cetesb, 2012)). Although located within the same basin these two riverine urban stretches are heavily impacted by different sources such as domestic effluents and industrial waste disposal in Tietê and insecticides in Pinheiros (Cunha et al., 2011).  Table 2 in (Kayranli et al., 2010)) and high mountain tropical lakes bury average rates of 60 to 301 mg C•m −2 •d −1 (Alcocer et al., 2020). This roughly points to a background tendency of increasing carbon burial with decreasing latitude, in -albeit experiencing human activity intervention-primarily natural sediments. Estimated C burial rates ranged between 408 and 995 mg C•m −2 •d −1 in the USA man-made reservoirs located between latitudes 25˚N and 50˚N and longitudes 67˚W and 125˚W ( Figure 3D in (Clow et al., 2015)). There, only a tenuous (if any) latitude dependence but a much stronger longitude-carbon burial rates increasing from east to west -dependence can be noted. Ranking high in CB is Acton Lake, a hypereutrophic hard-water 2.5 km 2 lake constructed in 1957 at latitude 39˚N in southwestern Ohio USA, with 932 mg C•m −2 •d −1 (Knoll et al., 2013). This potentially shows how the CB latitude dependence can be unobservable when comparing constructed reservoirs of different trophic states and characteristics.
Carbon burial efficiency is defined and approached in more than one way. For instance, non-mineralized organic carbon burial efficiencies are better constrained through refinement of the power law that describes organic carbon oxidation by incorporating the exposure time of sediments to oxygen (Katsev & Crowe, 2015). More, due to lack of available data on organic matter settling rates, Alin & Johnson (2007) defined CBE as the fraction of primary production  Table S3). The median carbon burial efficiency of young tropical meso-oligotrophic reservoirs has a positive correlation ratio with latitude of 0.22 % per degree south. The data used for this estimate is from Manso Reservoir (15˚S; 6.3%) in Table 1 of Sikar et al. (2012) and the here reported 3.7% (Table S3).
The silica-tracer method was devised to obtain higher temporally resolved estimations of CB to compare with daily emissions of greenhouse gases.  Baroni et al. (2020).

Conclusion
A robust (R 2 = 0.99) inverse correlation of 17.5 mg C•m −2 •d −1 per ˚S between carbon burial rate and tropical latitude was found in young tropical man-fabricated meso-oligotrophic reservoirs situated between latitudes 3˚S and 15˚S. While carbon burial rate decreases with increasing latitudes, carbon burial efficiency (here defined as ratio total-organic-carbon-buried-in-sediment/total-organic-carbonlanded-on-sediment) increases with increasing latitude at 0.22% per ˚S. Finally, quantifying not only the carbon sink rates but also the circulating carbon will better constrain the carbon budget of man-made environments. Table S2. Carbon and silica concentrations measured in sediment samples, carbon burial rate and carbon and silica depositional rates measured at sampled sites during each of the four field campaigns.
Source: our own elaboration. A Initially assigned zero because no sediment sample was in the dredge after executing the sediment sampling procedure. While discussing zero values one of us-statistician J. P.P. Dias-made the rather disconcerting assertion that "zero values had to be measured", a condition with which we complied from there on. B Plugged with a low value measured upstream at site ARM12. C Median of 18 samples collected during this survey from 18 sites. D Site not measured because of boggled air logistics one day before survey commencement. Plugged with interpolated figure based on moving averages of measured sites. E Traps, whether tampered with or lost, were not found upon retrieval. Plugged with interpolated figure based on moving averages of measured sites. F Traps were not found, possibly carried away by the strong water flow. Plugged with interpolated figure based on moving averages of measured sites. G Trap was lost. H Below detection limit of the analytical balance. Journal of Geoscience and Environment Protection