Less biologically dynamic ocean margins during the Ice Ages

The marginal ocean supports half of the ocean productivity in the present day, partly driven by the efficient nutrient cycling in the shallow continental shelf. The shallow shelf allows sinking matter to reach the bottom quickly and the breakdown of organic matter in the shallow sediments returns nutrients immediately to the sunlit upper ocean. As a result, the nutrients supplied to the waters overlying the continental shelf drive multiple rapid cycles of productivity, sedimentation, and remineralization over the broad extent of shallow seafloor. In this highly productive environment, the high flux of organic matter to the coastal seabed drives sediment anoxia (low oxygen) conditions, under which nitrate is quickly converted to nitrogen gases, a process called denitrification, which is the major loss term of biologically available nitrogen in the ocean. Nitrogen loss from the ocean sediments is estimated to account for at least half of the total nitrogen loss in the ocean, with the remaining nitrogen loss occurring in the major oxygen minimum zones in the water column.

During the Ice Ages, because of the growth of continental ice sheet, the average sea level is lowered by about 120 m, converting the continental shelves into coastal land, removing much of this environment as a site of oceanic nitrogen loss. The greater mean depth and steeper profile of the seaward continental slope should render the slope far less efficient at returning the nutrients released from the sediments to the upper ocean. Thus, upon sea level lowering, the coastal environment would be less favorable as an environment for both coastal productivity and benthic N loss. However, there have been as yet no direct tests of this hypothesis.

We found strong evidence suggesting that the continental margins are likely much less biologically dynamic in the past ice ages. We use the stable isotopes of nitrogen recorded by the foraminifera fossils preserved in the South China Sea deep sediments to reconstruct changes in the nitrogen cycle over the last 860,000 years. Our data suggests that marine N2 fixation, the main nitrogen input to the ocean, is significantly reduced in the Ice Ages. The changes in nitrogen fixation are strongly correlated with sea level changes over the past eight ice age-to-interglacial cycles. This correlation is best explained by a strong response of N2 fixation to marine nitrogen losses in the shallow shelf sediments. In the interglacials when sea level is high, denitrification occurring on the highly productive shallow continental shelf removes nitrogen from the ocean. This yields a selective advantage to N2 fixers by increasing the occurrence of phosphorus-bearing, N deplete surface waters. And the situation is reversed in the glacials when sea level is lower.

Our study points to ice age to interglacial oscillations in the biogeochemical fluxes at and near the ocean margins that would have influenced the evolution of coastal species. The reduction in benthic N loss during ice ages implies a net decline in the organic matter supply to coastal ecosystems, especially those organisms that rely on the benthos. In part because of their extraordinarily high productivity and benthic activity, the modern continental shelves have tremendous importance for seafloor fauna, fish, and marine mammals. The reconstructed biogeochemical changes imply that these higher trophic levels would have suffered a notable decline in food supply during the low sea level stands of ice ages (Figure 1), potentially impacting the evolution and current characteristics of coastal species and ecosystems.

The paper, "Impact of glacial/interglacial sea level change on the ocean nitrogen cycle," appears on July 31, 2017, in Proceedings of the National Academy of Sciences of the United States of America (http://www.pnas.org/content/114/33/E6759.abstract).

Figure 1. During interglacial high sea level stands, organic matter decomposition on the shallow shelf promotes high coastal ocean productivity and rapid shelf denitrification. The denitrification, by consuming fixed nitrogen (N), causes the shelf water to have excess phosphorus (P). When this water is transported into the open SCS, phytoplankton growth draws down its nutrients, and its excess P causes N to become depleted before P. The availability of P in the absence of N enhances N2 fixation. The sea level-driven loss of the shallow shelf during glacials reduces productivity and sedimentary denitrification along the margin. The reduction in sedimentary denitrification rate is compensated by slower offshore N2 fixation.