42 research outputs found
Extreme lowering of deglacial seawater radiocarbon recorded by both epifaunal and infaunal benthic foraminifera in a wood-dated sediment core
For over a decade, oceanographers have debated the interpretation
and reliability of sediment microfossil records indicating extremely low
seawater radiocarbon (14C) during the last deglaciation â
observations that suggest a major disruption in marine carbon cycling
coincident with rising atmospheric CO2 concentrations. Possible
flaws in these records include poor age model controls, utilization of mixed
infaunal foraminifera species, and bioturbation. We have addressed these
concerns using a glacialâinterglacial record of epifaunal benthic
foraminifera 14C on an ideal sedimentary age model (wood calibrated
to atmosphere 14C). Our results affirm â with important caveats â
the fidelity of these microfossil archives and confirm previous observations
of highly depleted seawater 14C at intermediate depths in the
deglacial northeast Pacific.</p
Abyssal Atlantic circulation during the Last Glacial Maximum: Constraining the ratio between transport and vertical mixing
The oceanâs role in regulating atmospheric carbon dioxide on glacialâinterglacial timescales remains an
unresolved issue in paleoclimatology. Reduced mixing between deep water masses may have aided oceanic
storage of atmospheric CO_2 during the Last Glacial Maximum (LGM), but data supporting this idea have
remained elusive. The δ^(13)C of benthic foraminifera indicate the Atlantic Ocean was more chemically
stratified during the LGM, but the nonconservative nature of δ^(13)C complicates interpretation of the LGM
signal. Here we use benthic foraminiferal δ^(18)O as a conservative tracer to constrain the ratio of meridional
transport to vertical diffusivity in the deep Atlantic. Our calculations suggest that the ratio was at least twice
as large at the LGM. We speculate that the primary cause was reduced mixing between northern and
southern component waters, associated with movement of this water mass boundary away from the zone of
intense mixing near the seafloor. The shallower water mass boundary yields an order of magnitude increase
in the volume of southern component water, suggesting its residence time may have increased substantially.
Our analysis supports the idea that an expanded volume of Antarctic Bottom Water and limited vertical
mixing enhanced the abyssal oceanâs ability to trap carbon during glacial times
Southwest Atlantic water mass evolution during the last deglaciation
The rise in atmospheric CO2 during Heinrich Stadial 1 (HS1; 14.5â17.5âkyr B.P.) may have been driven by the release of carbon from the abyssal ocean. Model simulations suggest that windâdriven upwelling in the Southern Ocean can liberate 13Câdepleted carbon from the abyss, causing atmospheric CO2 to increase and the δ13C of CO2 to decrease. One prediction of the Southern Ocean hypothesis is that water mass tracers in the deep South Atlantic should register a circulation response early in the deglaciation. Here we test this idea using a depth transect of 12 cores from the Brazil Margin. We show that records below 2300âm remained 13Câdepleted until 15âkyr B.P. or later, indicating that the abyssal South Atlantic was an unlikely source of light carbon to the atmosphere during HS1. Benthic δ18O results are consistent with abyssal South Atlantic isolation until 15âkyr B.P., in contrast to shallower sites. The depth dependent timing of the δ18O signal suggests that correcting δ18O for ice volume is problematic on glacial terminations. New data from 2700 to 3000âm show that the deep SW Atlantic was isotopically distinct from the abyss during HS1. As a result, we find that midâdepth δ13C minima were most likely driven by an abrupt drop in δ13C of northern component water. Low δ13C at the Brazil Margin also coincided with an ~80â° decrease in Î14C. Our results are consistent with a weakening of the Atlantic meridional overturning circulation and point toward a northern hemisphere trigger for the initial rise in atmospheric CO2 during HS1.Key PointsDeep SW Atlantic was unlikely source of light carbon to atmosphere during HS1Midâdepth isotopic anomalies due to change in northern component waterNorthern component water had robust influence in South Atlantic during HS1Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111970/1/palo20190.pd
Integrated analysis of bacterial and microeukaryotic communities from differentially active mud volcanoes in the Gulf of Cadiz
The present study assesses the diversity and composition of sediment bacterial and microeukaryotic communities from deep-sea mud volcanoes (MVs) associated with strike-slip faults in the South-West Iberian Margin (SWIM). We used a 16S/18S rRNA gene based pyrosequencing approach to characterize and correlate the sediment bacterial and microeukaryotic communities from MVs with differing gas seep regimes and from an additional site with no apparent seeping activity. In general, our results showed significant compositional changes of bacterial and microeukaryotic communities in sampling sites with different seepage regimes. Sediment bacterial communities were enriched with Methylococcales (putative methanotrophs) but had lower abundances of Rhodospirillales, Nitrospirales and SAR202 in the more active MVs. Within microeukaryotic communities, members of the Lobosa (lobose amoebae) were enriched in more active MVs. We also showed a strong correlation between Methylococcales populations and lobose amoeba in active MVs. This study provides baseline information on the diversity and composition of bacterial and microeukaryotic communities in deep-sea MVs associated with strike-slip faults
Collapse of the California Current During Glacial Maxima Linked to Climate Change on Land
Time series of alkenone unsaturation indices gathered along the California margin reveal large (4° to 8°C) glacial-interglacial changes in sea surface temperature (SST) over the past 550,000 years. Interglacial times with SSTs equal to or exceeding that of the Holocene contain peak abundances in the pollen of redwood, the distinctive component of the temperate rainforest of the northwest coast of California. In the region now dominated by the California Current, SSTs warmed 10,000 to 15,000 years in advance of deglaciation at each of the past five glacial maxima. SSTs did not rise in advance of deglaciation south of the modern California Current front. Glacial warming along the California margin therefore is a regional signal of the weakening of the California Current during times when large ice sheets reorganized wind systems over the North Pacific. Both the timing and magnitude of the SST estimates suggest that the Devils Hole (Nevada) calcite record represents regional but not global paleotemperatures, and hence does not pose a fundamental challenge to the orbital (âMilankovitchâ) theory of the Ice Ages
Seafloor geomorphic manifestations of gas venting and shallow subbottom gas hydrate occurrences
High-resolution multibeam bathymetry data collected with an autonomous underwater vehicle (AUV) complemented by compressed high-intensity radar pulse (Chirp) profiles and remotely operated vehicle (ROV) observations and sediment sampling reveal a distinctive rough topography associated with seafloor gas venting and/or near-subsurface gas hydrate accumulations. The surveys provide 1 m bathymetric grids of deep-water gas venting sites along the best-known gas venting areas along the Pacific margin of North America, which is an unprecedented level of resolution. Patches of conspicuously rough seafloor that are tens of meters to hundreds of meters across and occur on larger seafloor topographic highs characterize seepage areas. Some patches are composed of multiple depressions that range from 1 to 100 m in diameter and are commonly up to 10 m deeper than the adjacent seafloor. Elevated mounds with relief of >10 m and fractured surfaces suggest that seafloor expansion also occurs. Ground truth observations show that these areas contain broken pavements of methane-derived authigenic carbonates with intervening topographic lows. Patterns seen in Chirp profiles, ROV observations, and core data suggest that the rough topography is produced by a combination of diagenetic alteration, focused erosion, and inflation of the seafloor. This characteristic texture allows previously unknown gas venting areas to be identified within these surveys. A conceptual model for the evolution of these features suggests that these morphologies develop slowly over protracted periods of slow seepage and shows the impact of gas venting and gas hydrate development on the seafloor morphology