How much did Glacial North Atlantic Water shoal?

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 29 (2014): 190-209, doi:10.1002/2013PA002557.Observations of δ13C and Cd/Ca from benthic foraminifera have been interpreted to reflect a shoaling of northern source waters by about 1000 m during the Last Glacial Maximum, with the degree of shoaling being significant enough for the water mass to be renamed Glacial North Atlantic Intermediate Water. These nutrient tracers, however, may not solely reflect changes in water mass distributions. To quantify the distribution of Glacial North Atlantic Water, we perform a glacial water mass decomposition where the sparsity of data, geometrical constraints, and nonconservative tracer effects are taken into account, and the extrapolation for the unknown water mass end-members is guided by the modern-day circulation. Under the assumption that the glacial sources of remineralized material are similar to that of the modern day, we find a steady solution consistent with 241 δ13C, 87 Cd/Ca, and 174 δ18O observations and their respective uncertainties. The water mass decomposition indicates that the core of Glacial North Atlantic Water shoals and southern source water extends in greater quantities into the abyssal North Atlantic, as previously inferred. The depth of the deep northern-southern water mass interface and the volume of North Atlantic Water, however, are not grossly different from that of the modern day. Under this scenario, the vertical structure of glacial δ13C and Cd/Ca is primarily due to the greater accumulation of nutrients in lower North Atlantic Water, which may be a signal of the hoarding of excess carbon from the atmosphere by the glacial Atlantic.G.G. is supported by NSF grants OIA-1124880 and OCE-1301907, and the WHOI Ocean and Climate Change Institute.2014-09-1

Atlantic warming since the little ice age

Author Posting. © Oceanography Society, 2019. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Gebbie, G. Atlantic warming since the little ice age. Oceanography, 32(1), (2019):220-230, doi:10.5670/oceanog.2019.151.Radiocarbon observations suggest that the deep Atlantic Ocean takes up to several centuries to fully respond to changes at the sea surface. Thus, the ocean’s memory is longer than the modern instrumental period of oceanography, and the determination of modern warming of the subsurface Atlantic requires information from paleoceanographic data sets. In particular, paleoceanographic proxy data compiled by the Ocean2k project indicate that there was a global cooling from the Medieval Warm Period to the Little Ice Age over the years 900−1800, followed by modern warming that began around 1850. An ocean simulation that is forced by a combined instrumental-​proxy reconstruction of surface temperatures over the last 2,000 years shows that the deep Atlantic continues to cool even after the surface starts warming. As a consequence of the multicentury surface climate history, the ocean simulation suggests that the deep Atlantic doesn’t take up as much heat during the modern warming era as the case where the ocean was in equilibrium at 1750. Both historical hydrographic observations and proxy records of the subsurface Atlantic are needed to determine whether the effects of the Little Ice Age did indeed persist well after the surface climate had already shifted to warmer conditions.The author thanks Peter Huybers for collaborating on the Common Era temperature evolution, Lars Henrik Smedsrud for the encouragement to write this manuscript and compute heat fluxes, and to Ellen Kappel, Paul Durack, and Alex Sen Gupta for their handling of the manuscript. GG is supported by the James E. and Barbara V. Moltz Fellowship and NSF OCE-1357121. Correspondence and requests for materials should be addressed to the author