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

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⁻¹</span> for the previous <span class="inline-formula">∼120</span> years was determined from the <span class="inline-formula">^240+239Pu</span> signatures and the excess <span class="inline-formula">²¹⁰Pb</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>¹³C</span> and <span class="inline-formula"><i>δ</i>¹⁵N</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>