The molecular distributions and isotopic ratios (δ¹³C and δD) of n-alkanes in sediments deposited during the last 35 cal kyr BP in Saci Lake, southeastern Amazonia, have been measured and interpreted for their paleoclimate information. A terrestrial origin for the odd carbon-numbered long-chain (>C₂₇) n-alkanes has been inferred from molecular distributions. A shift in the n-alkane δ¹³C values across the Pleistocene-Holocene boundary coincides with a climate-induced floral change previously deduced from palynological analyses. Dry conditions during the late Pleistocene and mid-Holocene were confirmed by several n-alkane indices used to estimate the availability of moisture and by larger δD values. High ACL values indicate relatively dry conditions from 29 to 18.2 cal kyr BP and again from 8.0 and 5.0 cal kyr BP with a decrease in precipitation and an increase of grasses. Changes in the n-alkane C-29 isotopic compositions (δD and δ¹³C) were observed during the Pleistocene-Holocene transition, coinciding with a floristic change induced by climatic conditions that is also observed in palynological results from the same lake. The n-alkane C-29 δ¹³C values remain between −33.2‰ and − 35.7‰, which is representative for n-alkanes derived from C₃ vegetation. These lines of evidence suggest a continuous gallery forest cover in the Saci Lake catchment for the last ~35 cal kyr BP in contrasted with the observed palynological changes which represent the dynamics of the vegetation on the upland areas. The n-alkane C-29 δD values are less negative in the Pleistocene and more negative during the Holocene in the lake record. These characteristics suggest more humid conditions during the Holocene than in the Pleistocene, probably associated with the South American Monsoon System (SAMS). However, an increase of approximately 10‰ occurs during the late Holocene, which coincides with the forest expansion around 5 cal kyr BP that is recorded in palynological data of Saci Lake and other records of the Amazon Forest. This change suggests that the D/H ratio is also influenced by the increase of the plant biomass, which would provide an enrichment of D in the isotopic signal as a function of regional moisture recycling caused by evapotranspiration.

已经测量和解释了亚马逊东南部 Saci 湖最后 35 cal kyr BP 沉积物中正构烷烃的分子分布和同位素比（δ 13 C 和 δD），以获取它们的古气候^信息。从分子分布中推断出奇数碳长链 (>C ₂₇ ) n -烷烃的陆地来源。跨越更新世-全新世边界的正烷烃 δ ¹³ C 值的变化与先前从孢粉分析推断的气候引起的花卉变化相吻合。更新世晚期和全新世中期的干旱条件得到了数个 n的证实。-烷烃指数用于估计水分的可用性和较大的 δD 值。高 ACL 值表明从 29 到 18.2 cal kyr BP 以及从 8.0 到 5.0 cal kyr BP 的相对干燥条件，降水减少，草增加。在更新世-全新世过渡期间观察到正烷烃 C-29 同位素组成（δD 和 δ ¹³ C）的变化，这与气候条件引起的植物区系变化相吻合，这在同一湖泊的孢粉学结果中也观察到。正构烷烃C-29 δ ¹³ C 值保持在 -33.2‰ 和 -35.7‰ 之间，这代表了衍生自 C _3的正构烷烃植被。这些证据表明，在最后一个约 35 cal kyr BP 的 Saci 湖流域中存在连续的廊道森林覆盖，而观察到的孢粉变化代表了高地地区植被的动态。n _- 烷烃 C-29 δD 值在更新世时的负值较小，而在湖泊记录中的全新世时负值较大。这些特征表明全新世期间的条件比更新世时期更潮湿，这可能与南美季风系统（SAMS）有关。然而，在全新世晚期发生了大约 10‰的增加，这与 Saci 湖的孢粉学数据和亚马逊森林的其他记录中记录的 5 cal kyr BP 周围的森林扩张相吻合。这种变化表明 D/H 比也受到植物生物量增加的影响，这将提供同位素信号中 D 的富集，作为蒸发蒸腾引起的区域水分循环的函数。