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

Abstract

Micropaleontological and geochemical analyses of a sediment core collected in the Laurentian Trough of the Gulf of St. Lawrence were carried out to reconstruct temporal variations in pelagic productivity and benthic environmental conditions. Dinoflagellate cyst assemblages reveal relatively stable pelagic productivity over the last two centuries. Similarly, geochemical (organic C, C org : N) and isotopic (d 13 C org , d 15 N) data reveal that organic matter fluxes to the seafloor have been relatively constant over the same period. In contrast, significant changes are recorded in the benthic foraminifer assemblages. A sediment surface peak in the abundance of Cassidulina laevigata and Brizalina subaenariensis is consistent with the recent record of oxygen depletion in the bottom water. A decrease in the relative abundance of Nonionellina labradorica, concomitant with a relatively higher occurrence of Oridorsalis umbonatus in the upper part of the core, reflects a significant warming of the bottom water. Changes in bottom-water properties are further constrained by a negative trend of the d 18 O in Bulimina exilis carbonate shells over the last century, corresponding to a warming of about 2uC. These results strongly suggest that the recent oxygen depletion in the bottom waters of the Gulf of St. Lawrence is due to changes in water masses that have led to increased bottom-water temperatures and, to some extent, a resultant increase in organic matter respiration rates.Eutrophication is often identified as the main cause of bottom-water hypoxia (, 2 mg L 21 or 62.5 mmol L 21 ) in coastal environments (Cloern 2001). In the Lower St. Lawrence Estuary (LSLE), direct measurements have documented a progressive depletion of dissolved oxygen (DO) in bottom waters (. 250 m), from , 125 to 60 mmol L 21 over the last 80 yr (Gilbert et al. 2005). These observations are consistent with results of micropaleontological and geochemical analyses of sediment cores that show clear evidence of bottom-water oxygen depletion since the 1960s (Thibodeau et al. 2006). Whereas one half to two thirds of the oxygen depletion has been ascribed to a change in ocean circulation in the northwest Atlantic and, thus, variations in the water properties (DO, temperature, salinity) of the bottom waters that enter the Gulf and St. Lawrence through Cabot Strait (Gilbert et al. 2005), the remainder has been speculatively associated with increased organic carbon (C org ) fluxes to the seafloor, possibly due to anthropogenic eutrophication (Thibodeau et al. 2006). Thibodeau et al. (2006) and Gilbert et al. (2007) have shown that increases in sales of agricultural fertilizers, especially nitrogen fertilizers, in the St. Lawrence River drainage basin are concurrent with the depletion of bottom-water oxygen concentrations in the LSLE. Finally, an increase in bottom-water temperatures (Gilbert et al. 2005), leading to increased respiration rates, could also explain the recent decrease of DO concentrations.To determine if eutrophication and the trend ...

Low oxygen events in the Laurentian Channel during the Holocene

“…Estimation of summer sea-surface temperature (°C) and salinity, seasonal extent of sea-ice cover (number of months per year with concentration N50%) and primary productivity (gC m −2 yr −1 ) from the application of MAT on dinocyst data. trend in the LCBW has been observed in the estuary (Thibodeau et al, 2010a) and the Gulf of St. Lawrence (Genovesi et al, 2011). In the top of core CR06-TCE, the amplitude of the δ 18 O c decrease (0.6‰) is similar to the ones measured in the LSLE (Thibodeau et al, 2010a) and in the Emerald Basin, on the continental margin off Nova Scotia (Keigwin et al, 2003).…”

“…The recent decrease in oxygenation of the bottom waters in the LSLE and Gulf of St. Lawrence is regionally characterized by the occurrence of the low-oxygen species Brizalina subaenariensis (Kaiho, 1994;Bruchert et al, 2000;Thibodeau et al, 2006;Genovesi et al, 2011). In the central part of the Gulf of St. Lawrence, the increase of B. subaenariensis is concomitant with an increase of Oridorsalis umbonatus, a species typically found in the warmer water of the North Atlantic (Bilodeau et al, 1994).…”

“…This trend could also be attributed to an increase in remineralization rate under warmer conditions. Genovesi et al (2011) calculated that the~2°C temperature increase in the LCBW could have intensified bacterial activity, and thus the organic matter remineralization rate, by 11 to 27%. Consequently, enhanced release of metabolic CO 2 would result in lower pH and thus, conditions favorable for calcium carbonate dissolution .…”

“…Whereas enhanced marine organic matter fluxes in the 1960s may have contributed to local oxygen consumption in the bottom water of the LSLE (Benoit et al, 2006;Thibodeau et al, 2006), a significant warming of about +1.7°C in bottom waters during the last century has probably played a major role in the development of regional hypoxia (Gilbert et al, 2005;Thibodeau et al, 2010a). A warming of the same amplitude was reconstructed from a sediment core in the Gulf of St. Lawrence (Genovesi et al, 2011). These data suggest that the last century was marked by a warming of bottom waters throughout the Laurentian Channel from the Cabot Strait to the lower estuary.…”

Oxygen in the deep Strait of Georgia, 1951–2009: The roles of mixing, deep‐water renewal, and remineralization of organic carbon

“…Genovesi et al (2011), for example, calculated that microbial respiration in the deep water of the Gulf of St. Lawrence could have increased by 10-32% over the last century due to a 2uC temperature increase. It is not clear what effect such an increase in microbial respiration would have on oxygen in the deep Strait of Georgia.…”