By dissecting deep-sea corals capable of living for thousands of years, scientists were able to trace a massive shift in the open Pacific Ocean ecosystem back to around 1850.

Deep-sea corals feed on material that falls from the upper regions the ocean and their skeletons are made from a hard protein material that encodes the chemical signatures from their food.

"They're like living sediment traps, recording long-term changes in the open ocean that we can't see any other way," said Matthew McCarthy, professor of ocean sciences at the University of California, Santa Cruz (UCSC) and co-author of the study published in the journal Nature.

The researchers combined carbon dating and a new method for analyzing nitrogen isotopes in proteins that together enabled them to piece together the last 1,000 years of the ocean's history. The results revealed a dramatic shift some 160 years ago in the source of nitrogen feeding the ocean's surface waters.

Nitrogen has two main sources in the ocean: dissolved nitrate, which is found in its greatest concentrations in the subsurface and deep water, and nitrogen fixation by specific microorganisms that co-author Owen Sherwood, who worked on the study as postdoctoral researcher at UCSC, called the "legumes of the sea." According to scientists' findings, around the time the light bulb was being invented and Harriet Tubman was helping slaves escape north, the North Pacific Subtropical Gyre was shifting from the former to the latter. 

"As a result of decreasing nitrogen inputs from subsurface water, the phytoplankton community at the base of the food web became increasingly dominated by nitrogen-fixing cyanobacteria, which are able to use the nitrogen gas absorbed by surface waters from the atmosphere," UCSC officials wrote in a statement.

This shift may have been the result of the North Pacific Subtropical Gyre expanding and becoming warmer, causing a more distinct layer of warm surface water to form over cooler subsurface water, the researchers hypothesize. Such stratification limits the amount of nutrients transferred to the surface via nutrient-rich water from below.

If this is the case, it would offer important context to observations over the past few decades of warming and expansion of the major mid-ocean subtropical gyres - trends that have been attributed to global warming.

"It seems that the change in nitrogen sources," McCarthy said, "and therefore possibly large-scale shifts in ocean conditions, switched on at the end of the Little Ice Age and it is still continuing today."