Nutrient regime shift in the western North Atlantic indicated by compound-specific δ ¹⁵ N of deep-sea gorgonian corals

Abstract

Despite the importance of the nitrogen (N) cycle on marine productivity, little is known about variability in N sources and cycling in the ocean in relation to natural and anthropogenic climate change. Beyond the last few decades of scientific observation, knowledge depends largely on proxy records derived from nitrogen stable isotopes ( δ 15 N) preserved in sediments and other bioarchives. Traditional bulk δ 15 N measurements, however, represent the combined influence of N source and subsequent trophic transfers, often confounding environmental interpretation. Recently, compound-specific analysis of individual amino acids ( δ 15 N-AA) has been shown as a means to deconvolve trophic level versus N source effects on the δ 15 N variability of bulk organic matter. Here, we demonstrate the first use of δ 15 N-AA in a paleoceanographic study, through analysis of annually secreted growth rings preserved in the organic endoskeletons of deep-sea gorgonian corals. In the Northwest Atlantic off Nova Scotia, coral δ 15 N is correlated with increasing presence of subtropical versus subpolar slope waters over the twentieth century. By using the new δ 15 N-AA approach to control for variable trophic processing, we are able to interpret coral bulk δ 15 N values as a proxy for nitrate source and, hence, slope water source partitioning. We conclude that the persistence of the warm, nutrient-rich regime since the early 1970s is largely unique in the context of the last approximately 1,800 yr. This evidence suggests that nutrient variability in this region is coordinated with recent changes in global climate and underscores the broad potential of δ 15 N-AA for paleoceanographic studies of the marine N cycle.

Recent changes in bottom water oxygenation and temperature in the Gulf of St. Lawrence: Micropaleontological and geochemical evidence

“…The historical measurements of temperatures are also compatible with d 18 O data in Bulimina exilis, which suggest a warming of at least 1.4uC and probably as much as 2.0uC since the turn of the century (, 1880-1910) to 2005. A significant warming in the bottom waters of the Laurentian Channel in the GSL and LSLE during the course of the last century appears unquestionable (Thibodeau et al 2010;Sherwood et al 2011). A potentially important consequence of this increase in bottom-water temperature is that it directly affects the organic matter remineralization rate in the water column and sediments.…”

Midwater zooplankton and suspended particle dynamics in the North Pacific Subtropical Gyre: A stable isotope perspective

“…In the literature, TP has also been calculated using different suites of Tr-AAs and Sr-AAs (Hannides et al 2009b;Sherwood et al 2011). We calculated a Tr-AA-and Sr-AA-based measure of trophic position as TP Tr-Sr 5 1 + (d 15 N Tr-AA 2 d 15 N Sr-AA 2 b)/ TEF, where b for this suite of Tr-AA and Sr-AA is also 3.4% and the TEF is 5.6% (derived from Chikaraishi et al [2009]).…”

Three decades of high-resolution coastal sea surface temperatures reveal more than warming

“…4a), which may be related with the weakening of the North Atlantic Subpolar Gyre 25 and the Labrador Current 26 . South from the Bay of Fundy, coastal waters have warmed in average at ~0.1 to 0.3 °C per decade (Fig.…”