Oceanographic dynamics and the end of the last interglacial in the subpolar North Atlantic

Abstract

The last interglacial interval was terminated by the inception of a long, progressive glaciation that is attributed to astronomically influenced changes in the seasonal distribution of sunlight over the earth. However, the feedbacks, internal dynamics, and global teleconnections associated with declining northern summer insolation remain incompletely understood. Here we show that a crucial early step in glacial inception involves the weakening of the subpolar gyre (SPG) circulation of the North Atlantic Ocean. Detailed new records of microfossil foraminifera abundance and stable isotope ratios in deep sea sediments from Ocean Drilling Program site 984 south of Iceland reveal repeated, progressive cold water-mass expansions into subpolar latitudes during the last peak interglacial interval, marine isotope substage 5e. These movements are expressed as a sequence of progressively extensive southward advances and subsequent retreats of a hydrographic boundary that may have been analogous to the modern Arctic front, and associated with rapid changes in the strength of the SPG. This persistent millennial-scale oceanographic oscillation accompanied a long-term cooling trend at a time of slowly declining northern summer insolation, providing an early link in the propagation of those insolation changes globally, and resulting in a rapid transition from extensive regional warmth to the dramatic instability of the subsequent ∼100 ka.gyre systems | cold anomalies | frontal zone | ocean circulation

Last interglacial temperature seasonality reconstructed from tropical Atlantic corals

“…These modelling efforts reveal that the mid-LIG increased SST seasonality in the Caribbean Sea results primarily from summer warming, contrary to the conclusion reached in Winter et al (2003). Our coral Sr/Ca records and time-slice simulations also further substantiate the transient modelling and hosing experiments (Felis et al, 2015) that demonstrate the insensitivity of tropical SST seasonality to the wider climate instabilities that perturbed the Atlantic realm during the LIG (Sirocko et al, 2005;Blanchon et al, 2009;O'Leary et al, 2013;Galaasen et al, 2014;Mokeddem et al, 2014). In summary, our results affirm the prevailing influence of orbital insolation changes on tropical SST seasonality.…”

“…The LIG was punctuated by profound changes and instabilities. However, as demonstrated by a water hosing experiment performed on model simulations of LIG climate and a coral proxy record (Felis et al, 2015), temperature seasonality in the tropical Atlantic was relatively insensitive to the influences of monumental ice sheet loss (Blanchon et al, 2009), sea level rise (O'Leary et al, 2013), rapid reorganisation of oceanic currents (Galaasen et al, 2014), cold events (Mokeddem et al, 2014) and abrupt climatic shifts (Sirocko et al, 2005).…”

Insights into North Atlantic deep water formation during the peak interglacial interval of Marine Isotope Stage 9 (MIS 9)

“…1). The study area is highly sensitive to water mass mixing and stratification (Read 2000) at the oceanic front that separates the warm subtropical water mass and cold northern sourced water mass (Ruddiman and McIntyre 1973;Ruddiman 1977;Oppo et al 2006;Mokeddem et al 2014). It is bathed at depth by the Iceland Sea Overflow Waters (ISOW) that contribute to the modern NADW (Oppo et al 2006(Oppo et al , 2015Thornalley et al 2013), and also sits within the core of the Glacial North Atlantic Intermediate Water (Oppo and Lehman 1993;Raymo et al 2004;Curry and Oppo 2005;Praetorius et al 2008).…”

“…It is bathed at depth by the Iceland Sea Overflow Waters (ISOW) that contribute to the modern NADW (Oppo et al 2006(Oppo et al , 2015Thornalley et al 2013), and also sits within the core of the Glacial North Atlantic Intermediate Water (Oppo and Lehman 1993;Raymo et al 2004;Curry and Oppo 2005;Praetorius et al 2008). Sediments at the site are a mixture of primarily fine-grained detrital silts, microfossil CaCO 3 , and variable but lesser amounts of biogenic opal and ice-rafted sand (Raymo et al 1997;Praetorius et al 2008;Mokeddem et al 2014). Accumulation rates vary, reaching maximum values over 30 cm/ka, and averaging ~14 cm/ka for the study interval of MIS 9, allowing centennial-scale resolution of oceanographic and climatic conditions at this location.…”

“…The absence of any significant NADW signal at the water depth of Site 984 during the climatic optimum contrasts sharply with subsequent interglacial peaks (MIS 5e and the Holocene). Despite the perturbed intermediate depth circulation, oceanic heat transport northeastward was 1 3 , 2003O'Brien et al 1995;Bond et al 1997;Hall et al 1998;Chapman and Shackleton 1999;Cortijo et al 1999;Bond 2001;Oppo et al 1997Oppo et al , 2001Oppo et al , 2006Shackleton et al 2002;Mayewski et al 2004;Hodell et al 2009;Voelker and de Abreu 2011;Mokeddem et al 2014, and references therein), yet comparatively little attention has been dedicated to MIS 9 and its peak warming, MIS 9e (Tzedakis et al 2004;Roucoux et al 2006;Desprat et al 2009;Voelker and de Abreu 2011).…”