Understanding of diachroneity: Palaeoenvironmental controls on dispersal of planktic foraminifera in the Plio-Pleistocene oceans

Macroevolution is the generation, proliferation and removal of species. The lack of physical barriers and large population size of most marine species should allow for rapid dispersal and colonization of new habitats following speciation. However, gene flow appears more limited than we would predict based on this analysis: diachronous first occurrence dates among and within ocean basins of many species of planktonic foraminifera suggest that population establishment is often temporarily restricted immediately after speciation. Recent work shows that even in a cosmopolitan marine diatom species, gene flow is restricted at an ocean basin scale. These observations indicate that (i) physical barriers to dispersion in the open ocean are stronger than commonly invoked and (ii) the identity of these barriers and their working mechanisms are poorly understood.

The proposed research will study environmental drivers of dispersal dynamics in the open ocean of the circum- Antarctic using planktonic foraminifera of Plio-Pleistocene age in deep sea sediments recovered during International Ocean Discovery Program (IODP) Expedition 383 in May-July 2019. The named researcher, Dr Anieke Brombacher, participated in Expedition 383 shipboard as a planktonic foraminifer biostratigrapher as a UK-IODP sponsored scientist.

Planktonic foraminifera are ideally suited for in-depth analysis of temporal dispersal dynamics because their high-resolution fossil record in marine sediments allows for reconstructions of population dynamics across space and through time. Using sediments recovered by IODP Expedition 383, we will study species dispersal and migration in the sub-polar Southern Pacific Ocean, a region that, via the Drake Passage, connects the Pacific and Atlantic Oceans but lies at the edge of many species' geographical ranges. Expedition 383 drilled six South Pacific sites well north of the Antarctic front, that are well-suited to producing extremely high-resolution sediment records undisturbed by major ice rafting. We will document foraminifera evolution at three of these sites and compare those records to geochemical reconstructions of variability in the Antarctic Circumpolar Current (ACC). Specifically, high-resolution Pliocene-Pleistocene sediment archives recovered from IODP Sites U1539, U1540 and U1541 will be used to study edge population dispersal through Drake Passage, a key high latitude ocean gateway connecting the Pacific and Atlantic Oceans in response to 40-kyr glacial cycles.

The Neogene planktonic foraminifera species Truncorotalia truncatulinoides, Globoconella puncticulata puncticuloides and Globoconella inflata all originated in the Southwest Pacific Ocean. However, G. puncticulata puncticuloides has not been recorded in any other ocean basin, and T. truncatulinoides and G. inflata do not appear in the Atlantic Ocean until 0.5-1.0 Myr after their first appearance in the tropical to mid-latitude Pacific despite the Drake Passage forming an obvious potential migration route. This observation suggests that the high latitudes presented an environmental barrier to the dispersal of temperate-water species early in their evolutionary history. Initial shipboard results from IODP 383 Sites U1539 - U1541, all located around 55 degrees S in the SW Pacific, show varying assemblage compositions across glacial-interglacial cycles. Glacial assemblages are dominated by the polar species Neogloboquadrina pachyderma but interglacial assemblages contain more temperate species including various Truncorotalia and Globoconella taxa. These initial findings suggest that interglacials potentially act to weaken barriers to dispersal to higher latitudes and facilitate adaptation there, a mechanism akin to 'island hoping'. To test this working hypothesis, high-resolution species counts, morphometric and stable isotope data are required to determine whether adaptation improves with exposure to ocean conditions in successive interglacials.

The proposed research will be of interest to the general public interested in climate change and its effect on life on Earth. During the expedition Dr Brombacher engaged in a variety of outreach activities during the expedition. She wrote regular blog posts for the official website of the JOIDES Resolution (https://joidesresolution.org/author/abrombacher/), participated in live broadcasts from the ship to institutions all over the world which were watched by over 3000 people (https://www.facebook.com/US.NSF/videos/1011106315762584/) and will feature in a full-length documentary on Exp. 383. She has also developed new educational material in collaboration with IODP to show primary and secondary school students how microfossils can be used to date oceanic sediments. All material is available to the public through the JOIDES Resolution website (https://joidesresolution.org/activities/finding-fossils/).