North Pacific subtropical gyre (NPSG) δ15N proxy records and their relationship to climate change. Graphic: Sherwood, et al., 2014

By Robert Scribbler
18 December 2013

( – During the terrible mass extinction event at the Permian-Triassic boundary about 250 million years ago nearly all life on Earth was snuffed out. The event, which geologists have dubbed “The Great Dying,” occurred during a period of rapid warming on the tail end of a long period of glaciation (see A Deadly Climb From Glaciation to Hothouse: Why the Permian-Triassic Extinction is Pertinent to Human Warming). According to reports by Dr. Peter Ward, a prominent geologist specializing in causes of previous mass extinctions, the Permian extinction was composed of three smaller extinction events occurring over the course of about 50,000 to 80,000 years which together wiped out 96% of all marine species and 70% of all land species. Ward’s book “Under a Green Sky,” in my view, together with Hansen’s seminal “Storms of My Grandchildren” provide an excellent if terrible rough allegory of the climate beast we seem to be in the process of awakening. [more]

Through The Looking Glass of The Great Dying: New Study Finds Ocean Stratification Proceeded Rapidly Over Past 150 Years

ABSTRACT: The North Pacific subtropical gyre (NPSG) plays a major part in the export of carbon and other nutrients to the deep ocean1. Primary production in the NPSG has increased in recent decades despite a reduction in nutrient supply to surface waters2, 3. It is thought that this apparent paradox can be explained by a shift in plankton community structure from mostly eukaryotes to mostly nitrogen-fixing prokaryotes2, 3, 4. It remains uncertain, however, whether the plankton community domain shift can be linked to cyclical climate variability or a long-term global warming trend5. Here we analyse records of bulk and amino-acid-specific 15N/14N isotopic ratios (δ15N) preserved in the skeletons of long-lived deep-sea proteinaceous corals collected from the Hawaiian archipelago; these isotopic records serve as a proxy for the source of nitrogen-supported export production through time. We find that the recent increase in nitrogen fixation is the continuation of a much larger, centennial-scale trend. After a millennium of relatively minor fluctuation, δ15N decreases between 1850 and the present. The total shift in δ15N of −2 per mil over this period is comparable to the total change in global mean sedimentary δ15N across the Pleistocene–Holocene transition, but it is happening an order of magnitude faster6. We use a steady-state model and find that the isotopic mass balance between nitrate and nitrogen fixation implies a 17 to 27 per cent increase in nitrogen fixation over this time period. A comparison with independent records7, 8 suggests that the increase in nitrogen fixation might be linked to Northern Hemisphere climate change since the end of the Little Ice Age.

Increasing subtropical North Pacific Ocean nitrogen fixation since the Little Ice Age



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