Organic matter molecular composition reveals the impact of sea level change on the evolution of coastal wetland ecosystem since the Last Glacial Maximum on the west coast of Bohai Sea

doi:10.13745/j.esf.sf.2025.3.21

Abstract

Abstract:

The coastal wetland ecosystem is one of the important ecosystems and is sensitive to environmental changes. Changes in sea level directly affect the material sources of coastal wetlands, thereby influencing the development of wetland ecosystems. However, the impact of long-term sea level change on coastal wetland ecosystems has not been fully understood due to the lack of reliable geological records. This article aims to explore the impact of Bohai sea level changes on the development of coastal wetland ecosystems since the Last Glacial Maximum (LGM; ~22000 a BP) based on sedimentary organic matter (SOM) molecular composition. This article analyzed the SOM molecular composition on the west coast of the Bohai Sea with ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry technology. The results show that the proportion of endogenous aliphatic compounds in the sediments was relatively high during the LGM. The sea level was averagely ~130 m lower than the modern and no wetlands around the modern coast of the Bohai Sea were developed during this period, suggesting that the region may have developed local water systems and lacustrine environments. The sea level rose rapidly since the Holocene. Two marine facies in the core of this article recorded two marine transgression events during ~7100-6900 a BP and ~6000-5650 a BP. The sea level was relatively constant since ~5650 a BP and gradually retreated thereafter, resulting in 6 ancient coastlines and 6 stages of lagoon and depressions. The results suggest that the SOM CHO-component is sensitive to environmental changes induced by sea level change. Variations in H/C and O/C ratios indicate that the coastal wetlands underwent terrestrial-marine-terrestrial transformations from ~8050 to 4850 a BP. The higher O/C and lower H/C ratios during ~8050 a BP and ~4850 a BP indicate that wetlands and lagoon depressions might have developed on the west coast of the Bohai Sea, and changes in the SOM can be mainly ascribed to changes in terrestrial, semi-aquatic vegetation and soil microbial communities. The proportion of endogenous aliphatic compounds was higher during ~5700 a BP, suggesting that coastal wetlands have transformed into marine environments and the SOM sources have shifted to plankton. Endogenous organic components account for 15.68% during ~1350 a BP, significantly higher than those during ~8050 a BP and ~4850 a BP, indicating that the wetland ecological evolution may have been disturbed by strong human activities. This article provides new evidence and perspectives for understanding the impact of sea level change on the development of coastal wetland ecosystems.

Key words: coastal wetland, sea level change, organic molecular composition, FT-ICR MS, ecological evolution

CLC Number:

X142
X171.1

WANG Xinyu, XU Hai, WANG Jing, YANG Yan, WANG Fu, Liu Cong-Qiang. Organic matter molecular composition reveals the impact of sea level change on the evolution of coastal wetland ecosystem since the Last Glacial Maximum on the west coast of Bohai Sea[J]. Earth Science Frontiers, 2025, 32(3): 320-333.

Figures/Tables 10

Fig.1 Sketch map of the core locations in the study area, and maximum extent of Holocene marine encroachment and location of palaeoshorelines. DC01, QX01 and QX03 are three parallel cores in the study area (adapted from [22]). The curves and data represent the position of the palaeoshorelines and their ages (adapted from [23]).

Fig.2 Lithology, sampling locations and age-depth relationship of core QX01. Modified after [8,22,27-28].

Table 1 Sedimentary organic matter molecular composition results of 5 samples from core QX01

样品编号	深度/ m	年代/ a BP	分子数	m/z_w	C_w	H_w	O_w	N_w	S_w	H/C_w	O/C_w	N/C_w	S/C_w	DBE_w	KMD_w	AI-mod_w
QX01-1.45	1.45	1 350	4 777	380.05	17.04	20.02	8.93	0.45	0.17	1.18	0.55	0.04	0.01	8.31	0.32	0.24
QX01-5.4	5.4	4 850	2 836	385.23	16.83	18.55	9.70	0.59	0.07	1.11	0.60	0.04	0.004	8.85	0.34	0.28
QX01-6.8	6.8	5 700	5 073	385.51	17.37	20.46	8.98	0.53	0.20	1.18	0.54	0.04	0.01	8.41	0.33	0.25
QX01-12.4	12.4	8 050	3 419	388.82	17.03	18.62	9.64	0.58	0.14	1.09	0.59	0.04	0.01	9.01	0.35	0.29
QX01-15.0	15.0	22 000	4 044	376.01	17.04	20.40	8.69	0.49	0.19	1.16	0.42	0.03	0.61	8.09	0.32	0.15

Table 2 The compound groups and elemental composition of SOM molecular composition of 5 samples from core QX01

样品编号	各元素组合类别占比/%				各物质组合类占比/%
样品编号	CHO	CHON	CHOS	CHONS	木质素类	单宁类	不饱和烃类	芳香族	脂质	蛋白质	碳水化合物
QX01-1.45	45.59	40.36	12.61	1.44	50.84	25.17	0.93	7.38	1.8	7.75	6.13
QX01-5.4	53.01	39.01	7.73	0.25	47.03	25.77	8.62	7.19	0.85	5.67	4.87
QX01-6.8	40.18	34.93	20.77	4.12	61.87	7.06	3.98	4.05	2.4	13.28	7.36
QX01-12.4	46.22	37.69	15.98	0.11	53.95	23.49	5.98	9.64	0.54	5.13	1.27
QX01-15.0	41.88	30.37	24.46	3.29	52.82	18.34	3.11	9.17	3.07	9.97	3.52

Fig.3 (a)Relative abundance of compounds corresponding to combinations of elements; (b)Relative abundance of compounds corresponding to each substance combination

Fig.4 Records of sea level change and organic molecular composition

Fig.5 Paleo-coastline changes and coastal wetland evolution in Bohai Bay. Blue areas such as the ‘ancient Bei Da Gang’ are areas of perennial waterlogging. Light blue areas are lagoon type, and depressions of various phases. Dark blue areas are sea water inundation areas. Solid and dotted lines represent palaeoshorelines, and data represent palaeoshorelines ages (modified after [43]).

Fig.6 Comparison of CHO fractions of three samples. Venn diagram (a) demonstrates the number of common and unique molecules in the three samples, with 1293 common molecules and 42, 21 and 553 unique molecules in the three samples, respectively. V-K diagram (b) demonstrates the distributions of common and unique components in the three samples.

Fig.7 Highly oxygenated aromatic compounds (AI-modw<0.67, O/C>0.4) in three samples. The red solid line is AI-modw=0.67. The black dashed line is O/C=0.4. The black circles mark land-based sources of organic matter (adapted from [21]).

Fig.8 (a)-(c)Distribution of sulfur compounds in the V-K diagrams of the three samples;(d)Relative abundance of sulfur compounds in different samples

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