Owen A. Sherwood et al., PNAS (2011), Nutrient regime shift in the western North Atlantic indicated by compound-specific δ15N of deep-sea gorgonian corals

Proceedings of the National Academy of Sciences, published online before print January 3, 2011; doi: 10.1073/pnas.1004904108

Nutrient regime shift in the western North Atlantic indicated by compound-specific δ¹⁵N of deep-sea gorgonian corals

1. Owen A. Sherwood^a,¹,², 
2. Moritz F. Lehmann^b, 
3. Carsten J. Schubert^c,
4. David B. Scott^d and 
5. Matthew D. McCarthy^e

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 (δ¹⁵N) preserved in sediments and other bioarchives. Traditional bulk δ¹⁵N 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 (δ¹⁵N-AA) has been shown as a means to deconvolve trophic level versus N source effects on the δ¹⁵N variability of bulk organic matter. Here, we demonstrate the first use of δ¹⁵N-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, coralδ¹⁵N is correlated with increasing presence of subtropical versus subpolar slope waters over the twentieth century. By using the new δ¹⁵N-AA approach to control for variable trophic processing, we are able to interpret coral bulk δ¹⁵N 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 δ¹⁵N-AA for paleoceanographic studies of the marine N cycle.

Link to abstract:  http://www.pnas.org/content/early/2010/12/27/1004904108.abstract

Link to free, open-access, full article:  http://www.pnas.org/content/early/2010/12/27/1004904108.full.pdf+html

Moritz Lehmann and others: a drastic change to a "warm water mode" occurred in the western North Atlantic in the early 1970s, changing dominance of the Labrador Current to the Gulf Stream

Corals provide evidence of changes to oceanic currents

by Alton Parrish, editor,  beforeitsnews.com, January 5, 2011

Examination of deep sea corals reveals that there have been drastic changes to oceanic currents in the western North Atlantic since the 1970s. The influence of the cold water Labrador Current, which is in periodic interchange with the warm Gulf Stream, has been decreasing continually since the 1970s. Occurring at the same time as global warming, this phenomenon is unique in the past 2000 years. These results are reported by researchers from the University of Basel and Eawag in the current edition of the scientific journal Proceedings of the National Academy of Sciences.

Annual growth rings of deep sea corals in ultraviolet light

Credit: A. Sherwood

One of the oldest known weather systems in the world is the North Atlantic Oscillation (NAO), the periodic variation of atmospheric pressure difference between the Azores and Iceland. It dictates not only whether the winters in Europe will be cold and dry or wet and warm, but also influences the oceanic currents in the North Atlantic. On the continental shelf off Nova Scotia, the NAO seems to control the interaction between different water masses. During positive phases, the oceanography of the northwest American continental shelf is dictated by a relatively warm water mass at 10 °C which is salt and nutrient rich, originating from the Gulf Stream. If the NAO is in a negative phase, the Labrador Current is dominant, a relatively cold water mass at 6 °C, which is nutrient scarce and originates from sub-polar regions.

Using new geochemical methods, an international team of researchers including the biogeochemists Prof. Moritz Lehmann (University of Basel) and Dr. Carsten Schubert (Eawag: Swiss Federal Institute of Aquatic Science and Technology) were able to prove that a drastic change to a «warm water mode» occurred in the western North Atlantic in the early 1970s. This change, the timing of which coincides with and may be directly related to global warming, is unique in the last 2000 years.

Corals record climate data
The researchers made use of the fact that water masses carry different nitrogen isotopic signatures(different ratios of the stable nitrogen isotopes 15N und 14N) depending on their origins. These signals are then recorded in the biomass of deep sea corals hundreds of metres below the surface that feed on sinking organic particles from above. The deep sea corals thus allow a reconstruction of the oceanic current ratios over the last few decades. An exact dating of the individual samples is possible due to the corals’ production of easily identifiable annual growth rings. The researchers were able to show a clear reduction in the 15N/14N ratio since 1970 which indicates that the role of the cold Labrador Current, with a higher 15N/14N ratio, is becoming less important.

Possible alternative bio-ecological or geochemical causes for such a change in the stable isotope ratio were able to be excluded by the researchers using component-specific nitrogen analyses of the corals. Depending on the food chain structure, changes occur in the 15N/14N ratio of specific amino acids in the corals’ individual annual growth rings. The nitrogen isotope signatures of the amino acids show that the food chain effect did not play a significant role at least since the 1970s.

Global warming with consequences
Isotopic analysis of fossil deep sea corals from the same region confirms that the nitrogen isotope ratios and thus the oceanic current situation have remained practically unchanged over the past 2000 years. This indicates that the oceanographic change in oceanic currents of this scale, which has been occurring since the 1970s, is a unique occurrence within the past 2000 years.

The researchers suspect there is a direct connection between the changes in the oceanic currents in the North Atlantic and global warming primarily caused by human activities.

Source: Universität Basel

Citation: Owen A. Sherwood, Moritz F. Lehmann, Carsten J. Schubert, B. Scott and Matthew D. McCarty. Nutrient regime shift in the western north Atlantic indicated by compound-specific δ15N of deep sea Gorgonian corals. PNAS, published online before print January 3, 2011; doi: 10.1073/pnas.1004904108

Link:  http://beforeitsnews.com/story/340/602/Corals_Provide_Evidence_Of_Changes_To_Oceanic_Currents.html

Carsten Schubert and others, PNAS: find "drastic" shift since the 1970s in the north Atlantic Ocean currents (Labrador Current and Gulfstream) that usually influence weather in the Northern Hemisphere

Scientists Find Drastic Shift In Atlantic Ocean Currents

by RedOrbit Staff & Wire Reports, January 4, 2011

Swiss researchers reported on Tuesday that they found evidence of a "drastic" shift since the 1970s in the north Atlantic Ocean currents that usually influence weather in the northern hemisphere.

View of the Labrador Current, eight transatlantic flight minutes (130 km) east of Belle Isle, taken on a flight from London to Chicago. Credit: Daniel Schwen/Wikipedia 

The team of biochemists and oceanographers from Switzerland, Canada and the U.S. detected changes in deep sea Atlantic corals that indicated the declining influence of the cold northern Labrador Current.

They said that change "since the early 1970s is largely unique in the context of the last approximately 1,800 years," and raised the prospect of a direct link with global warming.

The Labrador Current interacts with the warmer Gulfstream from the south.

They have a complex interaction with a climate pattern, the North Atlantic Oscillation, which has a dominant impact on weather in Europe and North America.

Scientists have pointed to a disruption or shift in the oscillation as an explanation for moist or harsh winters in Europe in recent years.

One of the five scientists, Carsten Schubert of the Swiss Federal Institute of Aquatic Sciences and Technology (EAWAG), told AFP that for nearly 2,000 years, the sub-polar Labrador Current off northern Canada and Newfoundland had been the dominant force.

However, that pattern appears to have been repeated only occasionally in recent decades.
"Now the southern current has taken over, it's really a drastic change," Schubert told AFP, pointing to the evidence of the shift towards warmer water in the northwest Atlantic.

Research was based on nitrogen isotope signatures in 700-year-old coral reefs on the ocean floor, which feed on sinking organic particles.

While water pushed by the Gulfstream is salty and rich in nutrients, the colder Arctic waters carried by the Labrador Current contains fewer nutrients.

Changes could be dated back because of the natural growth rings that are found in corals.

"The researchers suspect there is a direct connection between the changes in oceanic currents in the North Atlantic and global warming caused by human activities," said EAWAG in a statement.

The scientists published their study recently in the Proceedings of the National Academy of Sciences.

Link:  http://www.redorbit.com/news/science/1973421/scientists_find_drastic_shift_in_atlantic_ocean_currents/index.html