The effect of global warming on the establishment of mangroves in coastal Louisiana during the Holocene

Winter temperature and sea-level position are critical factors affecting the global distribution of mangroves and saltmarshes. The replacement of saltmarshes by mangroves is expected due to global warming, reflecting the long-term natural trends in the Holocene and anthropogenic impacts since the 20th century. We documented the Holocene history of wetlands dynamics in the boreal limits of the American mangroves, located at Bay Champagne, Louisiana (USA), by integrating sedimentological, palynological, geochemical (delta C-13 and C\N), X-ray fluorescence (XRF) data, and radiocarbon chronology from two sediment cores. The results indicated a freshwater lake environment with herbs and wetland ferns, aswell as C-3 terrestrial plants, between similar to 8100 and similar to 6500 cal yr BP. This environment shifted into a lagoon and saltmarshes having sedimentary organic matter sourced frommarine algae between similar to 6500 and similar to 1500 cal yr BP. In the final stage, washover sediments were deposited in the lagoon during the last similar to 1500 cal yr BP. Despite the increased marine influence over the last similar to 6500 cal yr BP, mangrove pollenwere not recorded between similar to 8100 and similar to 1500 cal yr BP, suggesting that mangroves were absent in the study area during that time interval. Historical evidence and a comparative analysis of ourmulti-proxy datawith othermangrove studies fromthe Gulf of Mexico, Caribbean, and eastern South America revealed a gradualmangrove expansion from tropical to subtropical coasts of South and North America during the mid-late Holocene. The mangrove colonies at their current boreal (29 degrees 09' N) and austral (28 degrees 29' S) limits were established in the early and mid-20th century, respectively. This mangrove dynamics on a continental scale suggests that the polewardmangrove migrationwas likely caused by the warming climate during the Holocene. More importantly, the industrial-erawarming has likely accelerated themangrove expansion, but it was not the primary force that drove the mangrove migration into temperate zones. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Global distribution of mangroves and saltmarshes are affected by winter temperature and sea-level position. Evidence from Louisiana, USA, shows a shift from freshwater lake to lagoon and saltmarshes between 8100 and 1500 years ago, with mangrove expansion over the past 6500 years suggesting a gradual migration towards temperate zones.