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Termination II. First, much of the dust demise occurs prior to the change in CO2, so if iron is the villain, a threshold value of its supply must be called upon above which productivity does not continue to increase. Second, the CO2 rise continues for some 4-5 kyr after the dust flux has fallen to close to zero. These problems may be solved if the increased iron supply in dust caused higher rates of nitrogen fixation during the glacial periods. In this case the residence time of oceanic nitrate of a few thousand years would enable decreasing productivity to be a global rather than a local phenomenon and would explain the slow rampup of atmospheric CO2.

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The isotopic composition of carbon, δ¹³C, in seawater is used in reconstructions of ocean circulation, marine productivity, air-sea gas exchange, and biosphere carbon storage. Here, a synthesis of δ¹³C measurements taken from foraminifera in marine sediment cores over the last 150 000 years is presented. The dataset comprises previously published and unpublished data from benthic and planktonic records throughout the global ocean. Data are placed on a common δ¹⁸O age scale suitable for examining orbital timescale variability but not millennial events, which are removed by a 10 ka filter. Error estimates account for the resolution and scatter of the original data, and uncertainty in the relationship between δ¹³C of calcite and of dissolved inorganic carbon (DIC) in seawater. This will assist comparison with δ¹³C of DIC output from models, which can be further improved using model outputs such as temperature, DIC concentration, and alkalinity to improve estimates of fractionation during calcite formation.

High global deep ocean δ¹³C, indicating isotopically heavy carbon, is obtained during Marine Isotope Stages (MIS) 1, 3, 5a, c and e, and low δ¹³C during MIS 2, 4 and 6, which are temperature minima, with larger amplitude variability in the Atlantic Ocean than the Pacific Ocean. This is likely to result from changes in biosphere carbon storage, modulated by changes in ocean circulation, productivity, and air-sea gas exchange. The North Atlantic vertical δ¹³C gradient is greater during temperature minima than temperature maxima, attributed to changes in the spatial extent of Atlantic source waters. There are insufficient data from shallower than 2500 m to obtain a coherent pattern in other ocean basins. The data synthesis indicates that basin-scale δ¹³C during the last interglacial (MIS 5e) is not clearly distinguishable from the Holocene (MIS 1) or from MIS 5a and 5c, despite significant differences in ice volume and atmospheric CO₂ concentration during these intervals. Similarly, MIS 6 is only distinguishable from MIS 2 or 4 due to globally lower δ¹³C values both in benthic and planktonic data. This result is obtained despite individual records showing differences between these intervals, indicating that care must be used in interpreting large scale signals from a small number of records

This is a repository copy of Controls on the barium isotope compositions of marine sediments.

The last deglaciation is characterized by a rapid sea-level rise and coeval abrupt environmental changes. The Barbados coral reef record suggests that this period has been punctuated by two brief intervals of accelerated melting (meltwater pulses, MWP), occurring at 14.08-13.61 ka and 11.4-11.1 ka (calendar years before present), that are superimposed on a smooth and continuous rise of sea level. Although their timing, magnitude, and even existence have been debated, those catastrophic sea-level rises are thought to have induced distinct reef drowning events. The reef response to sea-level and environmental changes during the last deglacial sea-level rise at Tahiti is reconstructed based on a chronological, sedimentological, and paleobiological study of cores drilled through the relict reef features on the modern forereef slopes during the Integrated Ocean Drilling Program Expedition 310, complemented by results on previous cores drilled through the Papeete reef. Reefs accreted continuously between 16 and 10 ka, mostly through aggradational processes, at growth rates averaging 10 mm yr -1 . No cessation of reef growth, even temporary, has been evidenced during this period at Tahiti. Changes in the composition of coralgal assemblages coincide with abrupt variations in reef growth rates and characterize the response of the upward-growing reef pile to nonmonotonous sea-level rise and coeval environmental changes. The sea-level jump during MWP 1A, 16 ± 2 m of magnitude in ~350 yr, induced the retrogradation of shallow-water coral assemblages, gradual deepening, and incipient reef drowning. The Tahiti reef record does not support the occurrence of an abrupt reef drowning event coinciding with a sea-level pulse of ~15 m, and implies an apparent rise of 40 mm yr -1 during the time interval corresponding to MWP 1B at Barbados.

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