Where were the Caribbean mangroves during the Last Glacial Maximum? A microtopographical approach

Valentí Rull A

doi:10.1101/2025.01.13.632686

bioRxivJan 13, 2025PaleontologyPreprint

Where were the Caribbean mangroves during the Last Glacial Maximum? A microtopographical approach

Valentí Rull A iD

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Abstract

During the Last Glacial Maximum (LGM), when global sea levels dropped by [~]132 m, the Caribbean continental shelf was fully exposed, which drastically reduced the flat topographical habitat necessary for mangrove growth. It has been proposed that mangroves survived in flat microsites beyond the shelf break and later expanded from these microrefugia to their current distribution after the LGM. However, this hypothesis remains untested. This study aims to identify potential refugia by locating flat areas around the -132 m isobath using Global Multiresolution Topography (GMRT) images. A significant [~]200-km-long potential refugium was identified on the northern Trinidad (NT) shelf, along with several scattered kilometer-scale microrefugia near the Cariaco Basin (CB) in northeastern Venezuela. Additionally, two isolated prospective microrefugia were detected in northern Colombia (NC) and western Hispaniola (WH). Bioclimatically, all these potential LGM refugia were suitable for mangrove growth. The remaining LGM Caribbean coasts were considered unsuitable for mangrove growth. The NT refugium, along with the CB microrefugia, may have served as the primary sources for subsequent mangrove expansion. This expansion was likely facilitated by postglacial sea-level rise and the SE-NE Caribbean Current (CC), which would have acted as a major agent for propagule dispersal. This microtopographical survey not only supports the microrefugial hypothesis but also narrows the focus to the most promising areas, significantly reducing the time, effort and resources required for future seismic and coring campaigns.

In-depth summary

Claude Opus 5.5 · xhigh effort · 8 Nov 2026 · from the full text of version 3 (14 Mar 2025) · CC BY 4.0

Meter-scale bathymetry around the -132 m glacial shoreline identifies a candidate ~200-km mangrove refugium on the northern Trinidad shelf, plus km-scale microrefugia off Venezuela, Colombia and Hispaniola.

At the Last Glacial Maximum (LGM) sea-level lowstand, Caribbean shorelines sat beyond the shelf break on steep slopes unsuitable for mangroves, and with no in situ LGM mangrove record the microrefugial survival hypothesis was untested. This work maps flat sites near the LGM sea level to check whether that hypothesis is topographically plausible and to prioritize targets for seismic and coring campaigns.

Key results

ShownThe northern Trinidad (NT) shelf has the largest topographically suitable zone and is proposed as the primary LGM mangrove refugium. Its flat areas are widest in the centre and narrow to the east and west, where the coast becomes steeper.
Prospective flat areas span ~200 km. Central profile NTC: upper (-128 m) and lower (-133 m) mangrove limits nearly 6 km apart, mean slope 0.09%. Eastern NTE: 0.41%. Western NTW: limits <200 m apart, slope 3.05%, about 25 times the global mean (0.12%) and seven times the 0.45% maximum. · 3.2. Meso- and microtopography; Abstract
ShownAt the LGM lowstand, almost every Caribbean coastline lay beyond the shelf break on steep continental slopes. Topographically suitable habitat survived in only four zones: northern Trinidad (NT), Cariaco Basin (CB), northern Colombia (NC) and western Hispaniola (WH).
The -132 m sea level was offshore of the shelf break in all profiles. Exposed shelf reached more than 100 km offshore in Cuba, northeastern Venezuela/Trinidad and Nicaragua. Across 30 screened zones of 3° x 5°, the -128 to -133 m isobaths outside the four zones formed thin lines, which indicate steep coasts or cliffs. · 2.2. Bathymetric image analysis; 3.1. Macrotopography; 3.2. Meso- and microtopography
ShownSmall, scattered sites in the Cariaco Basin, northern Colombia and western Hispaniola meet mangrove slope criteria and qualify as microrefugia. The Hispaniola site is uncertain because it is a closed basin.
Cariaco Basin spots, each <10 km wide: 0.15% (CBN), 0.20% (CBS), 0.24% (CBW, with steeper central parts). Northern Colombia: 0.17% (NCS), 0.28% (NCN). Western Hispaniola: 0.23% to 0.36%, but it is a closed basin (likely a lagoon) of unknown salinity. All fall within the global range of 0.01% to 0.45%. · 3.2. Meso- and microtopography
SuggestedEstimated LGM temperatures at the candidate refugia stay above the lower limits of the main mangrove genera, and present rainfall lies within the broad range mangroves tolerate, so the author concludes climate did not limit survival there.
Adjacent coasts now average about 25°C to 28°C. Applying 5-6°C of Neotropical LGM cooling and a lapse-rate correction for the 132-m drop gives about 20°C to 24°C, above the lower limits for Rhizophora (19.2°C) and Avicennia (13.5°C). Current rainfall is 750 to 2050 mm/yr; mangroves tolerate <100 to >3000 mm/yr. · Conclusions and discussion; Tables
ClaimedAfter the LGM, mangroves probably recolonized the Caribbean westward from the NT and CB refugia, with propagules carried by the Caribbean Current as sea level rose.
No quantitative test. The argument rests on present-day circulation (the Northern Equatorial and Guyana currents feed the ESE-NW Caribbean Current) and on cited isotopic and model evidence that postglacial circulation resembled today's. Atlantic sources carried by the Guyana Current are not ruled out. · Conclusions and discussion; Abstract

Methods, data and reproducibility

Global Multi-Resolution Topography (GMRT) bathymetry was read at three scales. The -132 m contour delimited the exposed LGM shelf. Meter-scale isobaths from -128 to -133 m and land-sea profiles were then screened for flat areas with slopes inside the global mangrove range (0.01% to 0.45%).

GMRT bathymetry of the Caribbean between 8° and 23° N and 60° to 90° W, divided into zones of 3° x 5° for mesotopographic screening · n = 30 zones
Regional land-sea elevation profiles across shelf and slope off Mexico, Nicaragua, Panama, Colombia, Venezuela, Cuba, Jamaica, Dominican Republic, Anguilla and Martinique
Microtopographic profiles at candidate sites (NTW, NTC, NTE, CBN, CBS, CBW, NCS, NCN, WH) between the -128 and -133 m isobaths
Global mangrove elevation, inland extent and slope ranges compiled by Ellison et al. (2024), used as the reference gradient · mangrove trees (global)
1991-2020 coastal climate normals (mean annual temperature and precipitation) from Climatic Research Unit data mapped by the World Bank
Map of present Caribbean mangrove cover (Bunting et al. 2022) from NASA Landsat 5-TM imagery · mangrove trees

Methods & tools: Global Multi-Resolution Topography (GMRT) v4.3.0, GeoMapApp 3.7.4, Isobath (bathymetric contour) spacing analysis, Land-sea elevation profiling, Climatic Research Unit (CRU) climate data, World Bank Climate Knowledge Portal, Temperature lapse-rate correction (Martin & Fahey 2014), NASA Landsat 5-TM mangrove map (Bunting et al. 2022)
Data & accessions: http://www.geomapapp.org/https://earthobservatory.nasa.gov/images/47427/mapping-mangroves-by-satellite https://www.uea.ac.uk/groups-and-centres/climatic-research-unit https://climateknowledgeportal.worldbank.org/
Reagents & environment: GMRT version 4.3.0 viewed in GeoMapApp 3.7.4 (Lamont-Doherty Earth Observatory, Columbia University; Creative Commons Attribution Noncommercial Share Alike 3.0 license).
Reproducibility gaps: The data statement says only that the data are shown in the manuscript. No contour exports, profile coordinates or analysis files are deposited, and the zone-by-zone mesotopographic maps are only in the Supplementary Material.

Limitations (6)

A flat area near -132 m is only a minimum condition for LGM mangrove persistence. Salinity, river discharge and sediment or nutrient supply were not assessed and require marine coring and paleoecological surveys. · stated by the authors
Environmental niche modeling was left out of scope, though the author notes it could refine each refugium. LGM precipitation could not be estimated region-wide because paleoecological records vary strongly in space, so rainfall suitability rests on the broad tolerance of mangroves today. · stated by the authors
The western Hispaniola site is a closed basin, probably a coastal lagoon, of unknown water salinity, so its value as a mangrove microrefugium is uncertain. · stated by the authors
With current information, mangrove sources outside the Caribbean, such as Atlantic coasts supplying propagules via the Guyana Current, cannot be excluded as colonization origins. · stated by the authors
Present-day bathymetry stands in for LGM topography, with no correction for post-LGM sedimentation, subsidence, tectonics or regional isostatic sea-level differences. These could shift where flat areas lie at -132 m and change their slopes. · noted in this summary
Sites were picked by visual reading of isobath spacing and a few hand-placed profiles. There is no systematic slope mapping and no sensitivity test across the -130 to -134 m range, so smaller suitable sites may have been missed. · noted in this summary

Read this if you study Quaternary mangrove biogeography or Caribbean paleoecology and want bathymetry-based candidate sites for coring that could find Last Glacial Maximum mangrove refugia.