scholarly journals Causes and timing of recurring subarctic Pacific hypoxia

2021 ◽  
Vol 7 (23) ◽  
pp. eabg2906
Author(s):  
Karla P. Knudson ◽  
Ana Christina Ravelo ◽  
Ivano W. Aiello ◽  
Christina P. Knudson ◽  
Michelle K. Drake ◽  
...  
Keyword(s):  
Laminated Sediments ◽  
Last Deglaciation ◽  
Glacial Cycles ◽  
Sea Levels ◽  
Subarctic Pacific ◽  
High Productivity ◽  
The Past ◽  
Continental Shelves ◽  
Pacific Studies ◽  
The Last Deglaciation

Several North Pacific studies of the last deglaciation show hypoxia throughout the ocean margins and attribute this phenomenon to the effects of abrupt warming and meltwater inputs. Yet, because of the lack of long records spanning multiple glacial cycles and deglaciation events, it is unclear whether deoxygenation was a regular occurrence of warming events and whether deglaciation and/or other conditions promoted hypoxia throughout time. Here, subarctic Pacific laminated sediments from the past 1.2 million years demonstrate that hypoxic events recurred throughout the Pleistocene as episodes of highly productive phytoplankton growth and were generally associated with interglacial climates, high sea levels, and enhanced nitrate utilization—but not with deglaciations. We suggest that hypoxia was typically stimulated by high productivity from iron fertilization facilitated by redox-remobilized iron from flooded continental shelves.

scholarly journals The Meridional Shift of the Midlatitude Westerlies over Arid Central Asia during the Past 21 000 Years Based on the TraCE-21ka Simulations

2020 ◽  
Vol 33 (17) ◽  
pp. 7455-7478
Author(s):  
Nanxuan Jiang ◽  
Qing Yan ◽  
Zhiqing Xu ◽  
Jian Shi ◽  
Ran Zhang
Keyword(s):  
Indian Ocean ◽  
North Atlantic ◽  
North Atlantic Ocean ◽  
Last Deglaciation ◽  
The Past ◽  
The North ◽  
External Forcings ◽  
The Indian Ocean ◽  
The Last Deglaciation ◽  
The North Atlantic

AbstractTo advance our knowledge of the response of midlatitude westerlies to various external forcings, we investigate the meridional shift of midlatitude westerlies over arid central Asia (ACA) during the past 21 000 years, which experienced more varied forcings than the present day based on a set of transient simulations. Our results suggest that the evolution of midlatitude westerlies over ACA and driving factors vary with time and across seasons. In spring, the location of midlatitude westerlies over ACA oscillates largely during the last deglaciation, driven by meltwater fluxes and continental ice sheets, and then shows a long-term equatorward shift during the Holocene controlled by orbital insolation. In summer, orbital insolation dominates the meridional shift of midlatitude westerlies, with poleward and equatorward migration during the last deglaciation and the Holocene, respectively. From a thermodynamic perspective, variations in zonal winds are linked with the meridional temperature gradient based on the thermal wind relationship. From a dynamic perspective, variations in midlatitude westerlies are mainly induced by anomalous sea surface temperatures over the Indian Ocean through the Matsuno–Gill response and over the North Atlantic Ocean by the propagation of Rossby waves, or both, but their relative importance varies across forcings. Additionally, the modeled meridional shift of midlatitude westerlies is broadly consistent with geological evidence, although model–data discrepancies still exist. Overall, our study provides a possible scenario for a meridional shift of midlatitude westerlies over ACA in response to various external forcings during the past 21 000 years and highlights important roles of both the Indian Ocean and the North Atlantic Ocean in regulating Asian westerlies, which may shed light on the behavior of westerlies in the future.

scholarly journals The Mysterious 14C Decline

Radiocarbon ◽  
2009 ◽  
Vol 51 (1) ◽  
pp. 109-119 ◽  
Author(s):  
Wallace Broecker
Keyword(s):  
Age Differences ◽  
Radiocarbon Dating ◽  
Progressive Increase ◽  
Time Interval ◽  
Last Deglaciation ◽  
Polar Ice ◽  
Radiocarbon Age ◽  
Surface Ocean ◽  
The Past ◽  
The Last Deglaciation

Fundamental to the field of radiocarbon dating is not only the establishment of the temporal record of the calendar age-radiocarbon age offsets but also the development of an understanding of their cause. Although part of the decline in the magnitude of this offset over the past 40,000 can be explained by a drop in 14C production rate associated with a progressive increase in the strength of the Earth's magnetic shielding, it is clear that changes in the distribution of 14C among the Earth's active carbon reservoirs are also required. In particular, the steep 15% decline in the 14C to C ratio in atmospheric CO2 and surface ocean ΣCO2, which occurred in a 3 kyr-duration interval marking the onset of the last deglaciation, appears to require that a very large amount (at least 5000 gigatons) of 14C-deficient carbon was transferred to or within the ocean during this time interval. As no chemical or stable isotope anomaly associated with this injection appears in either the marine sediment or polar ice records, this injection must involve a transfer within the ocean (i.e. a mixing of 2 ocean reservoirs, one depleted in 14C and the other enriched in 14C). Although evidence for the existence of a salt-stabilized glacial-age abyssal ocean reservoir exists, a search based on benthic-planktic age differences and 13C measurements appears to place a limit on its size well below that required to account for the steep 14C decline.

scholarly journals Global acceleration in rates of vegetation change over the past 18,000 years

Science ◽  
2021 ◽  
Vol 372 (6544) ◽  
pp. 860-864
Author(s):  
Ondřej Mottl ◽  
Suzette G. A. Flantua ◽  
Kuber P. Bhatta ◽  
Vivian A. Felde ◽  
Thomas Giesecke ◽  
...  
Keyword(s):  
Vegetation Change ◽  
Terrestrial Ecosystems ◽  
Past Century ◽  
Last Deglaciation ◽  
The Past ◽  
Rates Of Change ◽  
Biodiversity Change ◽  
Human Effects ◽  
The Last Deglaciation ◽  
Global Vegetation

Global vegetation over the past 18,000 years has been transformed first by the climate changes that accompanied the last deglaciation and again by increasing human pressures; however, the magnitude and patterns of rates of vegetation change are poorly understood globally. Using a compilation of 1181 fossil pollen sequences and newly developed statistical methods, we detect a worldwide acceleration in the rates of vegetation compositional change beginning between 4.6 and 2.9 thousand years ago that is globally unprecedented over the past 18,000 years in both magnitude and extent. Late Holocene rates of change equal or exceed the deglacial rates for all continents, which suggests that the scale of human effects on terrestrial ecosystems exceeds even the climate-driven transformations of the last deglaciation. The acceleration of biodiversity change demonstrated in ecological datasets from the past century began millennia ago.

scholarly journals Laminated sediments in the Bering Sea reveal atmospheric teleconnections to Greenland climate on millennial to decadal timescales during the last deglaciation

2014 ◽  
Vol 10 (3) ◽  
pp. 2467-2518 ◽  
Author(s):  
H. Kuehn ◽  
L. Lembke-Jene ◽  
R. Gersonde ◽  
O. Esper ◽  
F. Lamy ◽  
...  
Keyword(s):  
Bering Sea ◽  
North Pacific ◽  
North Pacific Ocean ◽  
Ice Core ◽  
Laminated Sediments ◽  
Temperature Threshold ◽  
Last Deglaciation ◽  
The North ◽  
Decadal Scale ◽  
The Last Deglaciation

Abstract. During the last glacial termination, the upper North Pacific Ocean underwent dramatic and rapid changes in oxygenation that lead to the transient intensification of Oxygen Minimum Zones (OMZs), recorded by the widespread occurrence of laminated sediments on circum-Pacific continental margins. We present a new laminated sediment record from the mid-depth (1100 m) northern Bering Sea margin that provides insight into these deglacial OMZ maxima with exceptional, decadal-scale detail. Combined ultrahigh-resolution micro-XRF data and sediment facies analysis of laminae reveals an alternation between predominantly terrigenous and diatom-dominated opal sedimentation. The diatomaceous laminae are interpreted to represent spring/summer productivity events that occur at the retreating sea ice margin. We identified five laminated sections in the deglacial part of our site. Laminae counts were carried out on these sections and correlated to the Bølling–Allerød and Preboreal phases in North Greenland Ice Core (NGRIP) oxygen isotope record, indicating an annual deposition of individual laminae couplets. The observed rapid intra-decadal intensifications of anoxia, in particular within the Bølling–Allerød, are tightly coupled to short-term warm events through increases in regional biogenic productivity. By correlating the counted laminated sections with Bering Sea Surface Temperature records (SST) and NGRIP δ18O data, we propose a deglacial minimum SST of 6–7 °C for the preservation of laminae, which we call the deglacial temperature threshold for anoxia occurrence, a process that strongly implies a close atmospheric teleconnection between the North Pacific and North Atlantic regions. We suggest that concomitant increases in Bering Sea biogenic productivity, in combination with oxygen-poor waters entering the Being Sea, drove down oxygen concentrations to values below 0.1 mL L-1 and caused laminae preservation. Calculated benthic-planktic ventilation ages show no significant variations throughout the last deglaciation, indicating that changes in formation rates or differing sources of North Pacific mid-depth waters are not prime candidates for strengthening the OMZ at our site. The age models established by our correlation procedure allow to determine calendar age control points for the Bølling–Allerød and the Preboreal that are independent of the initial radiocarbon-based chronology. Resulting calculated reservoir ages are 875 yr during the Bølling–Allerød, and 910–770 yr for the Younger Dryas and the Preboreal, respectively.

scholarly journals An Ancient Meltwater Pulse Raised Sea Levels by 18 Meters

Eos ◽  
2021 ◽  
Vol 102 ◽  
Author(s):  
Tim Hornyak
Keyword(s):  
North America ◽  
Sea Level Rise ◽  
Sea Level ◽  
Last Deglaciation ◽  
Sea Levels ◽  
The Last Deglaciation ◽  
New Research

Meltwater pulse 1A, a period of rapid sea level rise after the last deglaciation, was powered by melting ice from North America and Scandinavia, according to new research.

scholarly journals CH<sub>4</sub> and N<sub>2</sub>O fluctuations during the penultimate deglaciation

2021 ◽  
Vol 17 (4) ◽  
pp. 1627-1643
Author(s):  
Loïc Schmidely ◽  
Christoph Nehrbass-Ahles ◽  
Jochen Schmitt ◽  
Juhyeong Han ◽  
Lucas Silva ◽  
...  
Keyword(s):  
Ice Core ◽  
Last Deglaciation ◽  
Glacial Cycle ◽  
Last Glacial Period ◽  
Modes Of Variability ◽  
The Past ◽  
Present Composite ◽  
The Last Deglaciation ◽  
Ch4 And N2o ◽  
The Last Glacial

Abstract. Deglaciations are characterized by the largest natural changes in methane (CH4) and nitrous oxide (N2O) concentrations of the past 800 000 years. Reconstructions of millennial- to centennial-scale variability within these periods are mostly restricted to the last deglaciation. In this study, we present composite records of CH4 and N2O concentrations from the EPICA Dome C ice core covering the penultimate deglaciation at temporal resolutions of ∼100 years. Our data permit the identification of centennial-scale fluctuations during the transition from glacial to interglacial levels. At ∼134 000 and ∼129 000 years before present (hereafter ka), both CH4 and N2O increased on centennial timescales. These abrupt rises are similar to the fluctuations associated with the Dansgaard–Oeschger events identified in the last glacial period. In addition, gradually rising N2O levels at ∼130 ka resemble a pattern of increasing N2O concentrations on millennial timescales characterizing the later part of Heinrich stadials. Overall, the events in CH4 and N2O during the penultimate deglaciation exhibit modes of variability that are also found during the last deglaciation and glacial cycle, suggesting that the processes leading to changes in emission during the transitions were similar but their timing differed.

Long-chain alkenone-inferred temperatures from the last deglaciation to the early Holocene recorded by annually laminated sediments of the maar lake Sihailongwan, northeastern China

The Holocene ◽  
2018 ◽  
Vol 28 (7) ◽  
pp. 1173-1180 ◽  
Author(s):  
Qing Sun ◽  
Guoqiang Chu ◽  
Manman Xie ◽  
Yuan Ling ◽  
Youliang Su ◽  
...  
Keyword(s):  
Temperature Increase ◽  
Growing Season ◽  
Northeastern China ◽  
Early Holocene ◽  
Laminated Sediments ◽  
Last Deglaciation ◽  
Temperature Changes ◽  
Maar Lake ◽  
The Last Deglaciation ◽  
Annually Laminated Sediments

Abrupt temperature changes during the last deglaciation are well recognized in Greenland ice cores and in deep-sea sediment records. On the continent of monsoonal Asia, however, only a few terrestrial temperature reconstructions extend to the Younger Dryas (YD). This hampers the understanding of how the Asian monsoon system responded to large-scale boundary changes in ice-sheet dynamics and reorganizations of atmospheric–oceanic circulation between the last deglaciation and the Holocene. Here, we report an alkenone-inferred temperature record from varved sediments of the maar lake Sihailongwan, northeastern China. Alkenone provides temperatures that represent the water temperature during the growing season when the lake is ice-free. Annually laminated sediments provide a reliable time control. Reconstructed temperatures reveal a distinctive pattern of variations during the last deglaciation: a temperature increase of 6°C at the onset of the Bølling–Allerød, two cold intervals (during the Older Dryas and the intra-Allerød cold period), a relatively minor temperature decrease of 1–3°C during the YD, and a rapid temperature increase of 4–5°C at the early Holocene. The reconstructed temperature records from Lake Sihailongwan and adjacent regions indicate that summer (or growing season) temperature changes were smaller than is evident in Greenland ice core records that are weighted toward winter.

scholarly journals Laminated sediments in the Bering Sea reveal atmospheric teleconnections to Greenland climate on millennial to decadal timescales during the last deglaciation

2014 ◽  
Vol 10 (6) ◽  
pp. 2215-2236 ◽  
Author(s):  
H. Kuehn ◽  
L. Lembke-Jene ◽  
R. Gersonde ◽  
O. Esper ◽  
F. Lamy ◽  
...  
Keyword(s):  
Bering Sea ◽  
North Pacific ◽  
North Pacific Ocean ◽  
Gulf Of Alaska ◽  
Laminated Sediments ◽  
Last Deglaciation ◽  
Export Production ◽  
Decadal Scale ◽  
The Last Deglaciation ◽  
The Bering Sea

Abstract. During the last glacial termination, the upper North Pacific Ocean underwent dramatic and rapid changes in oxygenation that lead to the transient intensification of oxygen minimum zones (OMZs), recorded by the widespread occurrence of laminated sediments on circum-Pacific continental margins. We present a new laminated sediment record from the mid-depth (1100 m) northern Bering Sea margin that provides insight into these deglacial OMZ maxima with exceptional, decadal-scale detail. Combined ultrahigh-resolution micro-X-ray-fluorescence (micro-XRF) data and sediment facies analysis of laminae reveal an alternation between predominantly terrigenous and diatom-dominated opal sedimentation. The diatomaceous laminae are interpreted to represent spring/summer productivity events related to the retreating sea ice margin. We identified five laminated sections in the deglacial part of our site. Lamina counts were carried out on these sections and correlated with the Bølling–Allerød and Preboreal phases in the North Greenland Ice Core (NGRIP) oxygen isotope record, indicating an annual deposition of individual lamina couplets (varves). The observed rapid decadal intensifications of anoxia, in particular within the Bølling–Allerød, are tightly coupled to short-term warm events through increases in regional export production. This dependence of laminae formation on warmer temperatures is underlined by a correlation with published Bering Sea sea surface temperature records and δ18O data of planktic foraminifera from the Gulf of Alaska. The rapidity of the observed changes strongly implies a close atmospheric teleconnection between North Pacific and North Atlantic regions. We suggest that concomitant increases in export production and subsequent remineralization of organic matter in the Bering Sea, in combination with oxygen-poor waters entering the Being Sea, drove down oxygen concentrations to values below 0.1 mL L−1 and caused laminae preservation. Calculated benthic–planktic ventilation ages show no significant variations throughout the last deglaciation, indicating that changes in formation rates or differing sources of North Pacific mid-depth waters are not prime candidates for strengthening the OMZ at our site. The age models established by our correlation procedure allow for the determination of calendar age control points for the Bølling–Allerød and the Preboreal that are independent of the initial radiocarbon-based chronology. Resulting surface reservoir ages range within 730–990 yr during the Bølling–Allerød, 800–1100 yr in the Younger Dryas, and 765–775 yr for the Preboreal.

scholarly journals Operation of the boreal peatland methane cycle across the past 16 k.y.

Geology ◽  
2019 ◽  
Vol 48 (1) ◽  
pp. 82-86 ◽  
Author(s):  
Yanhong Zheng ◽  
Zhengkun Fang ◽  
Tongyu Fan ◽  
Zhao Liu ◽  
Zhangzhang Wang ◽  
...  
Keyword(s):  
Direct Evidence ◽  
Ice Core ◽  
Ice Cores ◽  
Atmospheric Methane ◽  
Last Deglaciation ◽  
Methane Cycle ◽  
The Past ◽  
Stable Isotopic ◽  
The Last Deglaciation

Abstract The role of boreal wetlands in driving variations in atmospheric methane (CH4) concentrations across the last deglaciation (20–10 ka) and the Holocene is debated. Most studies infer the sources of atmospheric methane via ice-core records of methane concentration and its light stable isotopic composition. However, direct evidence for variations in the methane cycle from the wetlands themselves is relatively limited. Here, we used a suite of biomarker proxies to reconstruct the methane cycle in the Chinese Hani peat across the past 16 k.y. We found two periods of enhanced methanogenesis, at ca. 15–11 ka and ca. 10–6 ka, whereas weak methanogenesis characterized the late Holocene. These periods of enhanced methanogenesis relate to periods of high/increasing temperatures, supporting a temperature control on the wetland methane cycle. We found no biomarker evidence for intense methanotrophy throughout the past 16 k.y., and, contrary to previous studies, we found no clear control of hydrology on the peatland methane cycle. Although the onset of methanogenesis at Hani at ca. 15 ka coincided with a negative shift in methane δ13C in the ice cores, there is no consistent correlation between changes in the reconstructed methane cycle of the boreal Hani peat and atmospheric CH4 concentrations.

scholarly journals Tightened constraints on the time-lag between Antarctic temperature and CO<sub>2</sub> during the last deglaciation

2012 ◽  
Vol 8 (4) ◽  
pp. 1213-1221 ◽  
Author(s):  
J. B. Pedro ◽  
S. O. Rasmussen ◽  
T. D. van Ommen
Keyword(s):  
Ice Core ◽  
Time Lag ◽  
Ice Cores ◽  
Relative Timing ◽  
Last Deglaciation ◽  
Physical Processes ◽  
Atmospheric Concentration ◽  
Close Coupling ◽  
The Past ◽  
The Last Deglaciation

Abstract. Antarctic ice cores provide clear evidence of a close coupling between variations in Antarctic temperature and the atmospheric concentration of CO2 during the glacial/interglacial cycles of at least the past 800-thousand years. Precise information on the relative timing of the temperature and CO2 changes can assist in refining our understanding of the physical processes involved in this coupling. Here, we focus on the last deglaciation, 19 000 to 11 000 yr before present, during which CO2 concentrations increased by ~80 parts per million by volume and Antarctic temperature increased by ~10 °C. Utilising a recently developed proxy for regional Antarctic temperature, derived from five near-coastal ice cores and two ice core CO2 records with high dating precision, we show that the increase in CO2 likely lagged the increase in regional Antarctic temperature by less than 400 yr and that even a short lead of CO2 over temperature cannot be excluded. This result, consistent for both CO2 records, implies a faster coupling between temperature and CO2 than previous estimates, which had permitted up to millennial-scale lags.

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