Samples were collected during field campaigns from 2010 to 2012. In coastal water less than 5 metres deep, investigators used boat-deployed grab samplers or manually operated steel scoops. Grab samples provided at least 1 kilogram of wet sediment, from which the upper 0–5-centimetre layer was taken for analysis. At sites deeper than 5 metres, research vessels used box cores to collect sediment extending to 50 centimetres; the uppermost 5-centimetre mixed layer was subsampled as the surface fraction. Along the rivers, investigators collected 5-centimetre-thick floodplain and point-bar sediments with pre-cleaned stainless-steel scoops at intervals of 3–5 kilometres. The 104 fluvial samples came from the lower reaches and estuarine zones of the Yellow River, Liaohe, Haihe, Luanhe, Daliaohe, Dalinghe, Xiaolinghe, Daqinghe, Yongdingxinhe, Nanpaishuihe, Xiaoqinghe, Mihe, Bailanghe, Yuhe, Weihe and Jiaolaihe rivers. Differential GPS recorded the locations. Although the sediments represent modern depositional conditions, they are not a snapshot of a single moment: the upper marine layer may integrate years to decades depending on sedimentation, sediment mixing and bioturbation.

Each sediment sample was thoroughly homogenized before representative aliquots were taken for pollen and grain-size measurements, ensuring that both records came from the same material. Laboratory preparation removed carbonates with 10% HCl, dissolved silicates with 40% HF and removed humic material with 10% KOH in a heated water bath. Heavy-liquid flotation in zinc bromide, with a density of approximately 2.0 grams per millilitre, concentrated the pollen and spores. One tablet containing 27,637 ± 563 spores of exotic Lycopodium was added before chemical treatment so pollen concentrations could be calculated in grains per gram of dry sediment. Microscopists identified grains at 400× magnification, checking difficult specimens at 600× or under oil immersion at 1,000×. The target was at least 200 terrestrial pollen and spore grains per sample, although some samples fell below that threshold. Taxonomic identifications used the reference collection at the Qingdao Institute of Marine Geology and published pollen atlases. Pollen percentages were calculated from the terrestrial pollen sum, while fern-spore percentages used the total palynomorph sum; the Chenopodiaceae/Amaranthaceae-type designation was retained to match the archived count table and common Quaternary palynological usage.

Count quality varied across the 534-sample archive. Identified pollen grains per sample range from 59 to 1,530, with a mean of 397; the corresponding means are 406 grains in riverine samples and 395 in marine samples. Four riverine and six marine samples contain fewer than 100 identified grains, while 78% of all samples contain 300–600. Counts in that range provide comparatively stable percentages for dominant taxa, but users examining rare pollen must account for count-size effects. Low counts are concentrated mainly in Bohai Bay and at the Luanhe River estuary, where high sedimentation, coarse grain size or oxidation may dilute or degrade palynomorphs. Mean pollen concentration is higher in riverine samples, at 3,531 grains per gram, than in marine samples, at 2,419 grains per gram, a pattern consistent with proximity to terrestrial sources and dilution during marine sediment accumulation. Taxonomic richness ranges from 87 morphotypes in the riverine subset to 95 in the marine subset. The Daqinghe has the highest recorded riverine richness, with 52 morphotypes, and the Weihe the lowest, with 23, but these values are descriptive because they were not standardized by sample number or count size.

Marine assemblages are dominated by arboreal pollen, especially Pinus, Quercus and Betulaceae. Total arboreal pollen ranges from 12% to 98% and generally exceeds 50% in offshore samples. Pinus closely follows that overall pattern, reaching its highest values in the deeper Central Basin and Bohai Strait. Quercus averages 1.7% and reaches 10.6%, with higher values around the Laotieshan Channel and near-shore parts of northern Liaodong Bay and Laizhou Bay. Betulaceae averages 3.3% and reaches about 10.6%, showing a broadly similar distribution. Non-arboreal pollen ranges from 1.5% to 88% and is enriched in northern Liaodong Bay, Bohai Bay, Laizhou Bay and the northern Bohai Strait. Chenopodiaceae/Amaranthaceae-type pollen, the most abundant herbaceous group, has a mean abundance of 14% and a maximum of 59%. Artemisia, Poaceae, Typha and Cyperaceae also have predominantly near-shore distributions. Fern spores, led by Selaginella, Osmunda, Monoletes and Triletes, are relatively abundant in fine-grained Central Basin, Bohai Bay and Liaodong Bay deposits and reach 84% of the total palynomorph sum.

These spatial patterns are consistent with multiple transport and deposition processes rather than a direct representation of vegetation beside each sampling station. Bisaccate Pinus pollen is buoyant, has high production and preservation potential, and can travel through both the atmosphere and marine currents; it is consequently overrepresented in more distal marine sediments. Herbaceous pollen from floodplains, grasslands, salt marshes, wetlands and agricultural coastal plains is more closely associated with river discharge and short-distance coastal transport. Fine-grained deposits in low-energy parts of the basin favor the accumulation of fern spores, whereas coarse deposits and energetic channels can contain more locally sourced pollen. The paired design makes it possible to compare river assemblages with marine assemblages in one harmonized framework. It does not track individual grains or directly demonstrate that a particular assemblage is transformed into another, and some marine surface sediment may also contain reworked or relict material.

Exploratory redundancy analysis found that the retained marine predictors explained 29.6% of the variance in pollen composition, with an adjusted R² of 0.286, pseudo-F of 30.167 and a permutation p-value of 0.005. Water depth was the strongest retained predictor, with an exploratory λ1/λ2 ratio of 0.303, followed by longitude at 0.214 and latitude at 0.099; clay and silt made smaller contributions. Grain-size variables were screened for multicollinearity, with sand and mean grain size removed before the final model. The first two constrained axes accounted for 16.15% and 9.90% of total inertia, respectively, or 88.08% of constrained inertia. The remaining variation indicates that unmeasured factors such as seasonal winds, episodic floods, resuspension and preservation can also influence marine assemblages. Because latitude and longitude are spatial surrogates for environmental and vegetation gradients, they should not be interpreted as direct causal mechanisms.

The pollen-depth relationship was evaluated mainly within the 0–35-metre interval, which captures the principal basin-wide near-shore-to-offshore gradient and excludes the deeper, hydrodynamically distinctive Bohai Strait channel. Within this interval, Pinus percentages and total arboreal pollen increase significantly with water depth. Quercus, total herbaceous pollen, Chenopodiaceae/Amaranthaceae-type pollen and Artemisia decrease with depth. The rise in the arboreal total is therefore driven almost entirely by Pinus rather than by parallel increases in every tree taxon. Shallow sites around 0–15 metres show greater relative representation of river, floodplain, salt-marsh, wetland and agricultural-coastal inputs. Deeper central-basin sites around 15–35 metres contain a larger proportional contribution of buoyant saccate pollen as the fluvial signal is diluted and offshore transport, sorting and sediment focusing become more influential. This relationship is an internally consistent modern analogue, not a direct calibration of absolute palaeo-water depth.

The riverine ordination was significant, with pseudo-F = 6.393 and permutation p = 0.001. Geographic position, water discharge and pollen concentration together explained 20.53% of compositional variance, equivalent to an adjusted 17.3%. The first two constrained axes accounted for 9.32% and 6.09% of total inertia, respectively, representing 75.1% of constrained inertia. Longitude and latitude each had a single-predictor λ1/λ2 ratio of 0.234, ahead of water discharge at 0.129 and pollen concentration at 0.119. Sediment load and watershed area explained variance when fitted individually but were removed from the multivariable model because they were strongly collinear with other regional variables. The ordination shows structured spatial and hydrological associations while leaving most variation unexplained, as expected for pollen delivered through a multi-source system affected by wind, floods, preservation, reworking and other unmeasured processes.

For palaeoenvironmental reuse, the modern gradient is most useful for identifying assemblage tendencies, not assigning an absolute water depth or sea level. Shallow, fluvial-influenced assemblages generally have lower Pinus percentages and lower ratios of arboreal to non-arboreal pollen, Pinus to Artemisia, and bladder-bearing pollen to non-arboreal pollen. They also contain more Artemisia, Chenopodiaceae/Amaranthaceae-type pollen, Poaceae, Cyperaceae, aquatic pollen and freshwater algae. More distal assemblages show the opposite tendencies and may accompany finer offshore muds and open-marine indicators. These signals should be used together rather than individually and cross-checked against pollen concentration, grain size, sedimentary facies, dinoflagellate cysts, foraminifera, geochemical proxies and independent chronological control. Samples with low pollen sums or strong evidence of reworking should be analyzed separately or excluded from transfer-function development. Important limitations include limited investigation of relict deposits, river sampling concentrated in lower reaches and estuaries rather than entire catchments, incomplete representation of ecological gradients by coordinates and hydrology, and the compositional nature of pollen percentages. The dataset’s principal value is its documented modern land-to-sea comparison and explicit uncertainty framework, not a single proxy equation for past environmental change.

Subject of Research: Modern marine and fluvial pollen assemblages and their environmental associations in the Bohai Sea region

Article Title: A paired marine-fluvial modern pollen dataset from the Bohai Sea and its inflowing rivers, China

Article References: Li, J., Yang, S., Ye, S., Meng, X., Chen, X., Mei, X., Gao, M., Hao, Q., Cao, X., Chen, B., Yi, S., & Zhao, G. (2026). A paired marine-fluvial modern pollen dataset from the Bohai Sea and its inflowing rivers, China. Earth System Science Data, 18(10), 7661-7677. https://doi.org/10.5194/essd-18-7661-2026

Image Credits: AI Generated

DOI: 10.5194/essd-18-7661-2026

Keywords: Bohai Sea, modern pollen, fluvial sediments, marine sediments, pollen transport, palaeoenvironment, source-to-sink systems, Pinus pollen, grain size, China, paired, marine-fluvial