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Although many studies have attempted to reconstruct millennial‐scale hurricane patterns using various proxy‐based methods, it is still unclear what the most effective proxies are to identify storm surge deposits in different environmental settings. This study quantitatively compares the application of grain‐size, loss‐on‐ignition, stable isotopes, X‐ray fluorescence, and palynological proxies in paleotempestology from an organic‐rich environment in the Florida Everglades. The nonparametric tests indicate that only 9 among the 27 parameters (mean diameter, %water, %organic, %carbonate, Ca, Sr, Ca/Ti, Cl/Br, and marine microfossils) exhibited significant differences between storm‐surge and in situ deposits. The principal component analysis shows that five marine indicators (Sr, Ca, Ca/Ti, %Carbonate, and Marine microfossils) have the closest association with the allochthonous samples, while Cl/Br and Mz are the most sensitive proxies in low‐ and high‐energy environments, respectively. Moreover, organic geochemical proxies (e.g., δ13C and δ15N of bulk sedimentary organic matter) are ineffective for identifying storm‐surge deposits in organic‐rich mangrove environments. Plain Language Summary: To help objectively identify the occurrence of storm‐surge deposits in coastal sediment cores, this study uses a statistical approach to evaluate the effectiveness of 27 widely used sedimentological, organic and inorganic geochemical, and biological parameters in distinguishing allochthonous deposits from autochthonous deposits in an organic‐rich environment in the Florida Everglades. The results show that Ca, Sr, Ca/Ti, and marine microfossil are four of the most sensitive parameters to distinguish storm‐surge deposits in organic‐rich environments in south Florida. In addition, Cl/Br can be used to indicate storm surge inundation in further inland and lower‐energy environments. Moreover, organic geochemical proxies cannot effectively distinguish storm‐surge deposits from in situ deposits in mangrove‐dominated coastal zones. Future studies need to explore the use of more specific organic geochemical proxies (e.g., biomarker) in mangrove‐dominated environments and evaluate the effective proxies in detecting storm deposits from sandy, deltaic/estuarine, and nontropical coastal environments. Key Points: Ca, Sr, Ca/Ti, and marine microfossil are the most sensitive parameters to identify storm‐surge deposits in organic‐rich environmentsCl/Br can be used to indicate storm surge inundation in further inland and lower energy environmentsOrganic geochemical proxies cannot effectively distinguish storm‐surge deposits from in situ deposits in mangrove‐dominated coastal zones [ABSTRACT FROM AUTHOR]

In the face of rapidly changing climate and increasing natural and anthropogenic disturbances, the long-term dynamics of coastal ecosystems in relation to hurricanes and human activities has become a focus of research for ecologists and geomorphologists across the Gulf of Mexico. This study integrates diatom, remote sensing, geochemical, and hydrodynamic analyses to document the ecological and morphological development of the barrier-island and estuary-wetland complex bordering Galveston Bay, Texas, from the late Holocene to the Anthropocene. The multi-proxy dataset revealed two major ecological shifts between 1320 and 840 cal yr BP and between 230 cal yr BP and 2008 AD in Galveston Bay. The first shift was driven by the landfall of a backdoor hurricane at ∼1000 cal yr BP in combination with the aperiodic and intense long-shore current that caused the barrier complex to retreat over fluvial deposits while the Bolivar Peninsula was growing toward the west. These morphological changes resulted in a narrower inlet, hence, restricting the marine influence from entering the bay and converting Galveston Bay to a wave-dominated estuary that was shallower and fresher. The second shift was caused by increasing modification to the waterway and infrastructures since 1800 AD. As a result, rapid sediment accretion was induced near the North Jetty, forming the current Bolivar Flats and setting the stage for mangrove colonization. The warming climate and hurricanes in recent decades acted as catalysts that accelerated mangrove expansion on the Bolivar Peninsula. Our results imply that the proposed coastal barrier system will likely increase the sediment deficit, alter the biogeochemical environment, and impact the primary productivity in the bay. However, on a regional scale, more and more mangrove stands are expected to appear around estuaries and lagoons, and along man-made jetties and barriers along the Gulf. [Display omitted] • Galveston Bay experienced two major ecological shifts during the last 2000 years. • The shift from 1320 to 840 cal yr BP was driven by a hurricane landfall and the aperiodic and intense long-shore currents. • The second shift was caused by increasing modification to the waterway and infrastructures since 1800 AD. • The warming climate and storms in recent decades acted as catalysts that accelerated mangrove expansion in the study area. [ABSTRACT FROM AUTHOR]

Sea level rise and the associated inland shift of the marsh-mangrove ecotone in south Florida have raised many scientific and management concerns in recent years. Holocene paleoecological records can provide an important baseline to shed light on the long-term dynamics of vegetation changes across this ecotone in the past, which is needed to predict the future. In this study, we present palynological, X-ray fluorescence, and loss-on ignition data from four sedimentary cores recovered from a 20-km marine-to-freshwater transect along the Shark River Estuary, southwest Everglades, to document the patterns and processes of coastal vegetation changes in response to sea level rise since the mid-Holocene. Our record indicates that freshwater marsh progressively replaced marl prairies at the Shark River Estuary between 5700 and 4400 cal yr BP. As marine transgression continued, marine influence reached the threshold necessary for mangroves to establish at the current mouth of the Shark River Slough at 3800 cal yr BP. During the next 3000 years, although sea level rise in the Western North Atlantic slowed down to 0.4 mm/yr, a spatial and temporal gradient was evident as the marsh-mangrove ecotone shifted inland by 20 km from 3800 to 800 cal yr BP, accompanied by a gradual landward replacement of freshwater marsh by mangrove forest. If sea level continues to rise at 2.33 mm/yr in the 21st century in south Florida, it is possible that marine influence will reach the threshold for mangroves to establish in the central Everglades, and we could expect a much more aggressive mangrove encroachment toward the northern and interior parts of south Florida in the next few centuries. [ABSTRACT FROM AUTHOR]

• American mangroves reached their current range limits after the Industrial Era. • An over four-fold increase in mangrove cover was recorded after the 21st century. • Mangrove expansion at the boreal range limit is accelerated by warmer winters. • Mangrove expansion at the austral range limit is governed by habitat availability. Mangroves are expanding poleward in North and South America, but it is unclear whether such ecotonal-shift is an Industrial-Era phenomenon, or a recurring occurrence tied to Holocene climate variability. Our multi-proxy and remote sensing data from the U.S. Gulf Coast and southeastern Brazil suggest that the Industrial Era is the first time throughout the Holocene when mangroves were able to extend their distributions to the current boreal and austral range limits. A mangrove expansion of ∼1.3 ha (from 0.002 to 1.33 ha), 103 ha (from 23 to 126 ha), and 1.42 ha (from 0.39 to 1.81 ha) are documented at Bolivar Flats (Texas, U.S.A.), Mississippi River Delta (Louisiana, U.S.A.), and Apalachicola (Florida, U.S.A.), respectively, since the early 2000s, suggesting that mangrove expansion in North America is a 21st -century phenomenon and likely governed by warming winters. In contrast, the relatively slow mangrove expansion (from 96.1 to 106 ha between 2003 and 2019) at the austral range limit is controlled primarily by available habitats with suitable salinity and sediment substrate. [ABSTRACT FROM AUTHOR]

This study presents a 5200-year history of coastal environmental changes and shoreline fluctuations based on a multi-proxy sedimentary record developed from the eastern end of Louisiana's Chenier Plain. Palynological, loss-on ignition, grain-size, and X-ray fluorescence data for a 525 cm sedimentary core indicate five stages of ecosystem development in the Rockefeller Wildlife Refuge since the mid-Holocene. Between 5190 and 4240 cal yr BP, the relative sea-level was much lower than the present level, and our study area was occupied by a baldcypress swamp. When the Sale-Cypremont subdelta complex formed at a western position approximately 4600 years ago, our study area was transformed to a maritime forest about 4240–3440 cal yr BP, suggesting shoreline progradation. When the Mississippi River delta lobe switched to more easterly positions during the Teche, St. Bernard, Lafourche, and Plaquemine-Balize complexes, our study site was transformed back to a freshwater swamp between 3440 and 1520 cal yr BP and then to a coastal marsh between 1520 and 300 cal yr BP. After the Atchafalaya-Wax Lake subdelta complex formed at a more westerly position again, the modern-day beach form at 300 cal yr BP. These coastal ecosystem developments suggest that shoreline retreat started at ~3440 cal yr BP and persisted to the present day. In addition, evidence of at least four possible paleohurricane events was found during the late Holocene. Our multi-proxy record demonstrates that shoreline fluctuation in southwestern Louisiana was tied to the Mississippi River Delta Switch since the mid-Holocene. Shoreline progradation occurred during the Sale-Cypremont subdelta phase ~5200–3500 years ago, when the sediment supply was high. Retrogradation occurred after ~3500 cal yr BP when the Mississippi River subdelta complexes occupied more easterly position, resulting in diminished sediment supply. A significant increase in the delivery of water and total suspended sediment load from the Atchafalaya River to the Gulf of Mexico is needed in order to sustain the southwestern Louisiana coastal wetlands. Unlabelled Image • Louisiana's Chenier Plain was strongly influenced by the Mississippi-Atchafalaya River course changes. • Ecosystems changed from swamp to maritime forest during 5200-3440 cal yr BP, and then to beach/dune after 3440 cal yr BP. • Shoreline retrogradation began at ~3440 cal yr BP as the Mississippi River outlet switched from west to east. • Holocene shoreline fluctuations of the Chenier Plain were tied to the Mississippi River Delta Switch and RSLR [ABSTRACT FROM AUTHOR]

The projected warming trend in the 21st century is likely to alter the global distribution of mangroves. However, the migratory pattern of different mangrove species is still unclear, especially in the subtropical Brazilian littoral. This study utilizes pollen, diatom, and organic geochemistry to document the late Holocene morphological and ecological transformation in São Francisco do Sul Bay, the mangrove sub-range limit in south Brazil. This multi-proxy dataset indicates the establishment of saltmarshes and mangroves on muddy tidal flats at ∼1720 and ∼870 cal yr BP, and Laguncularia , Avicennia , and Rhizophora colonized the study area at ∼870, ∼390, and ∼70 cal yr BP, respectively. This stepwise succession of three mangrove species in São Francisco do Sul Bay, and poleward mangrove expansion toward the austral mangrove range limit, indicate that the migratory histories of different mangrove species were not synchronized during the late Holocene, and temperature is the primary climatic factor regulating the mangrove distribution in south Brazil. More importantly, the Medieval Climate Anomaly and the Current Warm Period were clearly registered in the pollen record, suggesting that the two climate anomalies likely facilitated the mangroves colonization into higher latitudes in south Brazil. Overall, our dataset indicates that mangrove expansion into more temperate zones will likely accelerate in South America. [Display omitted] • Temperature is the primary climatic factor regulating the mangrove distribution in South America. • Medieval Climate Anomaly and the Current Warm Period likely facilitated the poleward mangrove migration in south Brazil. • The migratory histories of different mangrove species in South America were not synchronized during the late Holocene. [ABSTRACT FROM AUTHOR]

The northern Gulf of Mexico shoreline is rapidly retreating, and coastal features of the abandoned Mississippi River delta complexes are eroding and subsiding. Bay Champagne is located in the Caminada-Moreau headland, a region located west of the currently active delta that has one of the highest rates of retreat and land loss globally. As a result, the shoreline at Bay Champagne has retreated more than two kilometers in the past 130 years, causing a gradual transition from a stable, circular freshwater lake to a frequently disturbed, semi-circular backbarrier lagoon. Analyses of clastic layers in a series of sediment cores collected at this site over the past decade indicate the lake was less perturbed in the past and has become increasingly more sensitive to marine incursion events caused by tropical cyclones as the shoreline retreated. Geochemical and pollen analyses of the cores also reveal profound changes in environmental and chemical conditions at Bay Champagne over the past century. Through relating stratigraphy to spatial changes observed from satellite imagery, this study establishes a timeline of the transition of Bay Champagne from an inland lake to a backbarrier environment, and demonstrated that these changes have not occurred at a constant rate. These results provide a case-study of changing local environmental conditions in response to rapid shoreline retreat, and therefore serve as a model for future coastal changes for Louisiana and other Gulf coastal areas. [Display omitted] • The shoreline at Bay Champagne (LA) retreated >2 km over the last century. • Maps and imagery detail a transition from freshwater lake to brackish lagoon. • Geochemical results from sediment cores also show a lake to lagoon transition. • Linking shore retreat with stratigraphy serves as a model for future changes. [ABSTRACT FROM AUTHOR]

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 (δ13C 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, as well as C 3 terrestrial plants, between ~8100 and ~6500 cal yr BP. This environment shifted into a lagoon and saltmarshes having sedimentary organic matter sourced from marine algae between ~6500 and ~1500 cal yr BP. In the final stage, washover sediments were deposited in the lagoon during the last ~1500 cal yr BP. Despite the increased marine influence over the last ~6500 cal yr BP, mangrove pollen were not recorded between ~8100 and ~1500 cal yr BP, suggesting that mangroves were absent in the study area during that time interval. Historical evidence and a comparative analysis of our multi-proxy data with other mangrove studies from the Gulf of Mexico, Caribbean, and eastern South America revealed a gradual mangrove expansion from tropical to subtropical coasts of South and North America during the mid-late Holocene. The mangrove colonies at their current boreal (29° 09′ N) and austral (28° 29′ S) limits were established in the early and mid-20th century, respectively. This mangrove dynamics on a continental scale suggests that the poleward mangrove migration was likely caused by the warming climate during the Holocene. More importantly, the industrial-era warming has likely accelerated the mangrove expansion, but it was not the primary force that drove the mangrove migration into temperate zones. • Subtropical mangroves were studied by pollen, facies, isotopes and X-ray fluorescence. • Lake shifted into a lagoon with saltmarsh and marine organic matter at 6.5 k cal yr BP. • Mangroves did not occur between ~8100 and ~1500 cal yr BP in the study area. • Gradual warming caused poleward mangrove migration during the mid-late Holocene. • The industrial-era warming has likely accelerated the mangrove expansion. [ABSTRACT FROM AUTHOR]