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      Historic carbon burial spike in an Amazon floodplain lake linked to riparian deforestation near Santarém, Brazil

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          Abstract

          <p><strong>Abstract.</strong> Forests along the Amazon Basin produce significant quantities of organic material, a portion of which is deposited in floodplain lakes. Deforestation in the watershed may then have potentially important effects on the carbon fluxes. In this study, a sediment core was extracted from an Amazon floodplain lake to examine the relationship between carbon burial and changing land cover and land use. Historical records from the 1930s and satellite data from the 1970s were used to calculate deforestation rates between 1930 to 1970 and 1970 to 2010 in four zones with different distances from the margins of the lake and its tributaries (100, 500, 1000 and 6000<span class="thinspace"></span><span class="inline-formula">m</span> buffers). A sediment accumulation rate of <span class="inline-formula">∼4</span><span class="thinspace"></span><span class="inline-formula">mm yr<sup>−1</sup></span> for the previous <span class="inline-formula">∼120</span> years was determined from the <span class="inline-formula"><sup>240+239</sup>Pu</span> signatures and the excess <span class="inline-formula"><sup>210</sup>Pb</span> method. The carbon burial rates ranged between 85 and 298<span class="thinspace"></span><span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="normal">g</mi><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">C</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">yr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="59pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="0a931835a80d05aad5f1e3c120469786"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-15-447-2018-ie00001.svg" width="59pt" height="15pt" src="bg-15-447-2018-ie00001.png"/></svg:svg></span></span>, with pulses of high carbon burial in the 1950s, originating from the forest vegetation as indicated by <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> and <span class="inline-formula"><i>δ</i><sup>15</sup>N</span> signatures. Our results revealed a potentially important spatial dependence of the organic carbon (OC) burial in Amazon lacustrine sediments in relation to deforestation rates in the catchment. These deforestation rates were more intense in the riparian vegetation (100<span class="thinspace"></span><span class="inline-formula">m</span> buffer) during the period 1930 to 1970 and the larger open water areas (500, 1000 and 6000<span class="thinspace"></span><span class="inline-formula">m</span> buffer) during 1970 to 2010. The continued removal of vegetation from the interior of the forest was not related to the peak of OC burial in the lake, but only the riparian deforestation which peaked during the 1950s. Therefore, this supports the conservation priority of riparian forests as an important management practice for Amazon flooded areas. Our findings suggest the importance of abrupt and temporary events in which some of the biomass released by deforestation, especially restricted to areas along open water edges, might reach the depositional environments in the floodplain of the Amazon Basin.</p>

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          Most cited references 30

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          Tropical deforestation and habitat fragmentation in the Amazon: satellite data from 1978 to 1988.

          Landsat satellite imagery covering the entire forested portion of the Brazilian Amazon Basin was used to measure, for 1978 and 1988, deforestation, fragmented forest, defined as areas less than 100 square kilometers surrounded by deforestation, and edge effects of 1 kilometer into forest from adjacent areas of deforestation. Tropical deforestation increased from 78,000 square kilometers in 1978 to 230,000 square kilometers in 1988 while tropical forest habitat, severely affected with respect to biological diversity, increased from 208,000 to 588,000 square kilometers. Although this rate of deforestation is lower than previous estimates, the effect on biological diversity is greater.
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            Episodic sediment accumulation on Amazonian flood plains influenced by El Niño/Southern Oscillation.

            Continental-scale rivers with a sandy bed sequester a significant proportion of their sediment load in flood plains. The spatial extent and depths of such deposits have been described, and flood-plain accumulation has been determined at decadal timescales, but it has not been possible to identify discrete events or to resolve deposition on near-annual timescales. Here we analyse (210)Pb activity profiles from sediment cores taken in the pristine Beni and Mamore river basins, which together comprise 720,000 km2 of the Amazon basin, to investigate sediment accumulation patterns in the Andean-Amazonian foreland. We find that in most locations, sediment stratigraphy is dominated by discrete packages of sediments of uniform age, which are typically 20-80 cm thick, with system-wide recurrence intervals of about 8 yr, indicating relatively rare episodic deposition events. Ocean temperature and stream flow records link these episodic events to rapidly rising floods associated with La Niña events, which debouch extraordinary volumes of sediments from the Andes. We conclude that transient processes driven by the El Niño/Southern Oscillation cycle control the formation of the Bolivian flood plains and modulate downstream delivery of sediments as well as associated carbon, nutrients and pollutants to the Amazon main stem.
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              Regionalization of methane emissions in the Amazon Basin with microwave remote sensing

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                Author and article information

                Journal
                Biogeosciences
                Biogeosciences
                Copernicus GmbH
                1726-4189
                2018
                January 22 2018
                : 15
                : 2
                : 447-455
                Article
                10.5194/bg-15-447-2018
                © 2018
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