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CO;2&rft_id=info:pmid/&rft_val_fmt=info:ofi/fmt:kev:mtx:journal&rft.genre=article&rft.aulast=Saenko&rft.aufirst=O. CO;2&id=pmid:&genre=article&aulast=Saenko&aufirst=O. All authors contributed to the interpretation and preparation of the final manuscript.

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This reversal of the ocean’s usual thermal stratification pre-dates the Bølling–Allerød warming and must have been associated with increased salinity at depth to preserve the static stability of the water column.

The depleted radiocarbon content of the warm and salty water mass implies a long-term disconnect from rapid surface exchanges, and, although uncertainties remain, is most consistent with a Southern Ocean source.

In most cases higher water tables, some perched, are indicated by the presence of mollisols and wet-meadow soils (aquolls), algal mats, or pond sediments, including dark gray to black diatomites, at 70 localities in the United States (Fig. Therefore, black mat is a general term that includes all such deposits, and some YD marls and diatomites are actually white. There are both younger and older black layers, but they do not appear to be widely distributed over the continent like the YD black mat, nor are they known to be associated with any major climatic perturbation as was the case with YD cooling.

These latter cases are included in the nearly 30 localities containing strata representing the Pleistocene-Holocene transition (Allerød-Younger Dryas-Holocene) that do not exhibit a black layer because of little or no interaction with ground water or are represented by white to gray diatomaceous strata of YD age (Fig. Map of the United States showing 57 locations listed in SI Table 2 where one or more sites with black mats of Younger Dryas age occur (filled circles).

Radiocarbon dating suggests that deposition onset for some black mats appears to have preceded the sudden onset of the YD, but these older ages likely result from mixing of organic carbon of different ages across the Z1–2 contact described later. for Brady soil development after evaluating 18 C ages at the tops and bottoms of the Brady paleosol that range from 7,940 ± 70 B. Nevertheless, all of the black mats in SI Table 2 clearly span the YD, and as discussed later, the end of the Pleistocene megafauna, the Rancholabrean termination, immediately preceded the onset of deposition of the black mat and YD cooling.