Negative stable carbon isotopic excursions have been observed throughout most of the mid‐depth (~1000–3000 m) Atlantic Ocean during Heinrich Stadial 1 (HS1) and the Younger Dryas (YD). Although there is an agreement that these mid‐depth excursions were in some way associated with a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), there is still no consensus on the precise mechanism(s). Here we present benthic stable carbon and oxygen isotopic (δ13C and δ18O) records from five cores from the western equatorial Atlantic (WEA). Together with published benthic isotopic records from nearby cores, we produced a WEA depth transect (~800–2500 m). We compare HS1 and YD data from this transect with data from previously published North and South Atlantic cores and demonstrate that the largest negative δ13C excursions occurred in the WEA during these times. Moreover, our benthic δ18O records require the presence of two water masses flowing from the Southern Ocean, bisected by a Northern Component Water (NCW). Given that δ18O is a conservative water mass tracer, we suggest that δ13C was decoupled from water mass composition and does not correspond to simple alternations between northern and southern sourced waters. Instead, δ13C behaved non‐conservatively during HS1 and the YD. Consistently with our new 231Pa/230Th record from the WEA transect, that allowed the reconstruction of AMOC strength, we hypothesize that the negative δ13C excursions reflect an increase in the residence time of NCW in response to a weakened AMOC, allowing for a marked accumulation of 13C‐depleted respired carbon at the mid‐depth WEA.
Seafloor methane release can significantly affect the global carbon cycle and climate. Appreciable quantities of methane are stored in continental margin sediments as shallow gas and hydrate deposits, and changes in pressure, temperature and/or bottom-currents can liberate significant amounts of this greenhouse gas. Understanding the spatial and temporal dynamics of marine methane deposits and their relationships to environmental change are critical for assessing past and future carbon cycle and climate change. Here we present foraminiferal stable carbon isotope and sediment mineralogy records suggesting for the first time that seafloor methane release occurred along the southern Brazilian margin during the last glacial period (40–20 cal ka BP). Our results show that shallow gas deposits on the southern Brazilian margin responded to glacial−interglacial paleoceanographic changes releasing methane due to the synergy of sea level lowstand, warmer bottom waters and vigorous bottom currents during the last glacial period. High sea level during the Holocene resulted in an upslope shift of the Brazil Current, cooling the bottom waters and reducing bottom current strength, reducing methane emissions from the southern Brazilian margin.
Abstract. The formation of the Paraíba do Sul river delta plain on the
coast of Rio de Janeiro state, Brazil, gave rise to diverse lagoons formed
under different sea level regimes and climate variations. Sedimentary core
lithology, organic matter geochemistry, and isotopic composition (δ13C and δ15N) were analyzed to interpret the
sedimentation of the paleoenvironment of the Lagoa Salgada carbonate system.
Different lithofacies reflect variations in the depositional environment.
The abundance of silt and clay between 5.8 and 3.7 kyr enhances the
interpretation of a transgressive system, which promoted the stagnation of
coarse sediment deposition due to coast drowning. Geochemistry data from
this period (5.8–3.7 kyr) suggest the dominance of a wet climate with
an increase of C3 plants and a marked dry event between 4.2 and 3.8 kyr. This
dryer event also matches with previously published records from around the
world, indicating a global event at 4.2 ka. Between 3.8 and 1.5 kyr,
Lagoa Salgada was isolated; sand and silt arrived at the system by erosion
with the retreat of the ocean and less fluvial drainage. Geochemistry from
this moment marks the changes to favorable conditions for microorganisms
active in the precipitation of carbonates, forming microbial mats and
stromatolites in the drier phase.
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