Late Holocene paleoenvironmental changes in Northeast Brazil recorded by organic matter in lacustrine sediments of Lake Boqueirão

Abstract

Northeast Brazil represents a key area in terms of Holocene environmental changes in South America due to its distinct semi-arid climate at near equatorial latitudes as well as to a wetter Early and Middle Holocene paleoclimate in comparison with other South America regions, which also experienced a reduced monsoon at that time. We studied the lacustrine sediments of Lake Boqueirão, which is located at 5°S latitude on the Brazilian Atlantic coast; it is directly influenced by displacement of the Intertropical Convergence Zone (ITCZ). A short (1m) core “Boqc0701” was collected at a 7.5m water depth during a seismic survey. Organic matter (OM) geochemical [Rock‐Eval Total Organic Carbon (TOC), Hydrogen Index (HI), Oxygen Index (OI)] and petrographical studies allowed us to distinguish five sedimentary intervals during the last 3kyr. Phase E (3000 to 2050calyr BP) presents the largest TOC, HI and OI fluctuations in the entire core. The most probable cause of these variations is a great instability in the lake level during this period. Phase D (2050 to 1830calyr BP) is marked by an increase in sedimentation rate and a higher contribution from a well-preserved OM with an algal origin; this was interpreted as a higher and more stable lake level. During phase C (1830 to 1550calyrs BP), low HI and high OI indicate the input of more oxidized, degraded and detrital OM, reflecting a lower lake level. In phase B (1550 to 1470calyr BP), the OM quality and quantity (HI and TOC) indicate an algal contribution. During phase A (1470 to 570calyr BP), the high stability of the proxies may be due to a deep environment where OM fluctuations are strongly buffered. These changes in lake level can be compared with other tropical South America high resolution records. A correspondence seems to exist between phases of low ENSO, cool North Atlantic Ocean and Boqueirão wet phases. We did not encounter this same pattern during the last 1050years, during which the 1050–570calyrs BP interval was generally wet (although probable fluctuations are buffered in our proxies) while the North Atlantic was warm (Mann et al., 2009). A prolonged dryness occurred in Lake Boqueirão from 570calyrs BP until recent decades. This phase corresponds to the Little Ice Age characterized by a cooler North Atlantic Ocean (Mann et al., 2009). These data indicate that the teleconnection pattern between the tropical Atlantic, ENSO and Northeast Brazil rainfalls has changed in the past.