Earth's climate underwent a fundamental change between 1250 and 700 thousand years ago, the mid-Pleistocene transition (MPT), when the dominant periodicity of climate cycles changed from 41 thousand ...to 100 thousand years in the absence of substantial change in orbital forcing. Over this time, an increase occurred in the amplitude of change of deep-ocean foraminiferal oxygen isotopic ratios, traditionally interpreted as defining the main rhythm of ice ages although containing large effects of changes in deep-ocean temperature. We have separated the effects of decreasing temperature and increasing global ice volume on oxygen isotope ratios. Our results suggest that the MPT was initiated by an abrupt increase in Antarctic ice volume 900 thousand years ago. We see no evidence of a pattern of gradual cooling, but near-freezing temperatures occur at every glacial maximum.
Abstract
While the ocean’s large-scale overturning circulation is thought to have been significantly different under the climatic conditions of the Last Glacial Maximum (LGM), the exact nature of the ...glacial circulation and its implications for global carbon cycling continue to be debated. Here we use a global array of ocean–atmosphere radiocarbon disequilibrium estimates to demonstrate a ∼689±53
14
C-yr increase in the average residence time of carbon in the deep ocean at the LGM. A predominantly southern-sourced abyssal overturning limb that was more isolated from its shallower northern counterparts is interpreted to have extended from the Southern Ocean, producing a widespread radiocarbon age maximum at mid-depths and depriving the deep ocean of a fast escape route for accumulating respired carbon. While the exact magnitude of the resulting carbon cycle impacts remains to be confirmed, the radiocarbon data suggest an increase in the efficiency of the biological carbon pump that could have accounted for as much as half of the glacial–interglacial CO
2
change.
Core MD04‐2822 from the Rockall Trough has apparent sedimentation rates of ∼ 1 m/kyr during the last deglaciation (Termination I). Component magnetization directions indicate a magnetic excursion at ...16.3 m depth in the core, corresponding to an age of 26.5 ka, implying an excursion duration of ∼350 years. Across Termination I, the mean grain size of sortable silt implies reduced bottom‐current velocity in the Younger Dryas and Heinrich Stadial (HS)−1A, and increased velocities during the Bølling‐Allerød warm period. Standard bulk magnetic parameters imply fining of magnetic grain size from the mid‐Younger Dryas (∼12 ka) until ∼ 8 ka. First‐order reversal curves (FORCs) were analyzed using ridge extraction to differentiate single domain (SD) from background (detrital) components. Principal component analysis (FORC‐PCA) was then used to discriminate three end members corresponding to SD, pseudo‐single domain (PSD), and multidomain (MD) magnetite. The fining of bulk magnetic grain size from 12 to 8 ka is due to reduction in concentration of detrital (PSD + MD) magnetite, superimposed on a relatively uniform concentration of SD magnetite produced by magnetotactic bacteria. The decrease in PSD+MD magnetite concentration from 12 to 8 ka is synchronized with increase in benthic δ13C, and with major (∼70 m) regional sea‐level rise, and may therefore be related to detrital sources on the shelf that had reduced influence as sea level rose, and to bottom‐water reorganization as Northern Source Water (NSW) replaced Southern Source Water (SSW).
Key Points
FORC‐PCA as a new magnetic grain‐size discriminator
Relationship of magnetic and sortable‐silt grain size proxies over Termination I
New putative magnetic excursion at 26.5 ka
The basis for, and use of, fine grain size parameters for inference of paleoflow speeds is reviewed here. The basis resides in data on deposited sediment taken in conjunction with flow speed ...measurements in the field, experimental data on suspended sediment transport and deposition, and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed. In the deep sea, sorting events occur under resuspension/deposition events in benthic storms. At flow speeds below 10–15 cm s−1, size in the noncohesive “sortable silt” (10–63 μm) range is controlled by selective deposition, whereas above that range, removal of finer material by winnowing also plays a role. The best particle size instruments to measure a flow speed–related grain size employ the settling velocity method, while laser diffraction sizers can yield misleading results because of particle shape effects. Potential problems, including source effects, downslope supply on continental margins, spatial variability of flow over bedforms, and influence of ice‐rafted detritus, are examined. A number of studies using the sortable silt flow speed proxy are reviewed, and inverse modeling of grain size distributions is examined. Outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed.
It has been proposed that the ventilation of the deep Pacific carbon pool was not significantly reduced during the last glacial period, posing a problem for canonical theories of glacial–interglacial ...CO2 change. However, using radiocarbon dates of marine tephra deposited off New Zealand, we show that deep- (>2000 m) and shallow sub-surface ocean–atmosphere 14C age offsets (i.e. ‘reservoir-’ or ‘ventilation’ ages) in the southwest Pacific increased by ∼1089 and 337 yrs respectively, reaching ∼2689 and ∼1037 yrs during the late glacial. A comparison with other radiocarbon data from the southern high-latitudes suggests that broadly similar changes were experienced right across the Southern Ocean. If, like today, the Southern Ocean was the main source of water to the glacial ocean interior, these observations would imply a significant change in the global radiocarbon inventory during the last glacial period, possibly equivalent to an increase in the average radiocarbon age >2 km of ∼700 yrs. Simple mass balance arguments and numerical model sensitivity tests suggest that such a change in the ocean's mean radiocarbon age would have had a major impact on the marine carbon inventory and atmospheric CO2, possibly accounting for nearly half of the glacial–interglacial CO2 change. If confirmed, these findings would underline the special role of high latitude shallow sub-surface mixing and air–sea gas exchange in regulating atmospheric CO2 during the late Pleistocene.
•Tephra provide new deglacial southwest Pacific radiocarbon ventilation ages.•Both surface- and deep reservoir ages increased during the last glacial.•Similar changes are seen across the sub-Antarctic Southern Ocean.•Major impacts on the marine/atmospheric radiocarbon/carbon inventories are implied.
There are climatically important ocean flow systems in high latitudes, for example the East and West Greenland and Labrador Currents and Nordic Sea overflows in the North, and Antarctic Circumpolar ...Current in the South, for which it would be useful to know history of flow strength. Most of the sediment records under these flows contain evidence of supply from glacial sources, which has led to the supposition that fine sediment records, which in other settings provide evidence of vigour of flow from the sortable silt proxy, are fatally contaminated by unsorted glacial silt. It is suggested here that if the fine fraction (<63 μm) has been transported and sorted, then it does not matter that it may have been released from icebergs, sea ice or meltwater plumes. Here we show that correlation between sortable silt mean and percentage provides a good indicator of whether a fine sediment record has been sufficiently well current-sorted to provide a reliable flow history. The running downcore correlation (rrun) (5 to 9-point depending on sampling interval) is found to be optimal, and a value of rrun < 0.5 is proposed as an indicator of sufficiently poor sorting to invalidate a section of mean size record. More than 40 grainsize records determined by laser particle sizers from over 30 core sites have been processed and examined for evidence of sorting. As expected, there is a tendency for poor sorting and unreliable records at points where the flow speed has decreased to very low values. There is no consistent relationship between the sorting of the fine fraction and the content of coarse ice-rafted debris (as long as the IRD fraction is not > 50%) because the two are not related. End member (EM) decomposition of several records yields variable results in terms of the relationship between EM ratios and grainsize parameters. Although such an approach can generate fine sediment parameters it does not provide a basis for deciding whether or not a record is acceptably current sorted and thus contains a valid flow speed proxy. Our proposed discrimination between current-sorted and unsorted fine fractions is applicable to all fine grained deposits, not only high-latitude deposits with coarse IRD.
Examples from East Greenland, Faroe Bank Channel, Gardar Drift show mainly well sorted signatures. Amounts of coarse IRD range up to 60% with only those >50% having a consistent impact on sortable silt mean size. With the exception of a Southern Ocean site on the Antarctic continental rise where half the record is poorly sorted, the silt mean data are sufficiently well sorted to provide credible flow speed histories. This bodes well for the extraction of such histories from climatically important high-latitude flows such as the East Greenland Current.
•Correlation between sortable silt parameters is current-sorting index for palaeoflow.•Running downcore correlation <0.5 indicates an invalid size record for flow history.•No consistent relation between fine fraction sorting and coarse IRD content.•End members (EM) do not show whether or not a record is acceptably current sorted.•EMs confuse by combining sediment components having differing depositional origins.
A condition for the existence of sediment waves under turbidity currents as antidunes with the requirement of a slope gradient ≥3.0×10−3 is deduced. Data show no such waves on slopes <2.5×10−3, but ...some contourite mudwaves occur on slopes as low as 4.4×10−4. However the latter also occur on slopes >3×10−3 so no clear distinction is possible. Where turbidity current channels cross sediment drifts, or geostrophic flows traverse turbidite fans, the origin of most mudwaves will need to be determined by reference to internal features and context. A key problem is deposition of mud as antidunes from turbidity currents where even the waning flow is probably well above the critical erosion velocity for a clear flow. Deposition must occur from high concentration flows well above clear water critical depositional stresses. Once a wavy bed is set up, subsequent deposition may occur via the lee-wave mechanism proposed for contourite waves under a gradient Froude Number >1. A steep angle (<45°) between crest and flow axes is typical of GF waves, which may be dunes or antidunes, whereas TC waves tend to be orthogonal, but data on this discriminant are sparse.
•Turbidity current sediment waves should (do) not exist on slopes <3×10−3.•Contourite mudwaves can exist on lower gradients but there is no clear distinction.•Turbidite antidune muds are deposited at stresses above critical for clear water.•Angles between mudwave crest and geostrophic flow directions are typically <45°.
Eight- to ten-point depth profiles (from 1200 to 4800
m water depth) of oxygen and carbon isotopic values derived from benthic foraminifera, averaged over selected times in the past 160
ka, are ...presented. The data are from 10 sediment cores off eastern New Zealand, mainly North Chatham Rise. This lies under the Deep Western Boundary Current in the Southwest Pacific and is the main point of entry for several water masses into the Pacific Ocean. The benthic isotopic profiles are related to the structure of water masses at present and inferred for the past. These have retained a constant structure of Lower Circumpolar Deep Water–Upper Circumpolar Deep Water/North Pacific Deep Water–Antarctic Intermediate Water with no apparent changes in the depths of water mass boundaries between glacial and interglacial states. Sortable silt particle size data for four cores are also examined to show that the vigour of the inflow to the Pacific, while variable, appears to have remained fairly constant on average. Among the lowest Last Glacial Maximum values of benthic
δ
13C in the world ocean (−1.03‰ based on
Cibicidoides wüllerstorfi) occurs here at ∼2200
m. Comparable values occur in the Atlantic sector of the Southern Ocean, while those from the rest of the Pacific are distinctly higher, confirming that the Southern Ocean was the source for the unventilated/nutrient-enriched water seen here. Oxygen and carbon isotopic data are compatible with a glacial cold deep water mass of high salinity, but lower nutrient content (or better ventilated), below ∼3500
m depth. This contrasts with the South Atlantic where unventilated/nutrient-enriched water extends all the way to the sea bed. Comparison with previous studies also suggests that the deeper reaches of the Antarctic Circumpolar Current below ∼3500
m are not homogeneous all around the Southern Ocean, with the Kerguelen Plateau and/or the Macquarie-Balleny Ridges posing barriers to the eastward spread of the deepest low-
δ
13C water out of the South Atlantic in glacials. These barriers, combined with inferred high density of bottom waters, restricted inter-basin exchange and allow three glacial domains dominated by bottom waters from Weddell Sea, Adelie Coast and Ross Sea to be defined. We suggest that the Ross Sea was the main source of the deep water entering the Pacific below ∼3500
m.
During the Last Glacial Maximum (LGM; ~20,000 years ago), the global ocean sequestered a large amount of carbon lost from the atmosphere and terrestrial biosphere. Suppressed CO
outgassing from the ...Southern Ocean is the prevailing explanation for this carbon sequestration. By contrast, the North Atlantic Ocean-a major conduit for atmospheric CO
transport to the ocean interior via the overturning circulation-has received much less attention. Here we demonstrate that North Atlantic carbon pump efficiency during the LGM was almost doubled relative to the Holocene. This is based on a novel proxy approach to estimate air-sea CO
exchange signals using combined carbonate ion and nutrient reconstructions for multiple sediment cores from the North Atlantic. Our data indicate that in tandem with Southern Ocean processes, enhanced North Atlantic CO
absorption contributed to lowering ice-age atmospheric CO
.
PDF
