In sediments, plant biomolecules (n-alkanes and fatty acids) serve as robust proxies in paleoenvironmental studies. Although the factors controlling preservation and isotopic composition of plant biomolecules are understood in soil systems, the effect of changes in depositional environments in land-sea interfaces, especially in hypersaline environments, is not so well constrained. In this study, we observed that in bed sediments, the plant biomolecules are preserved with carbon preference index (CPI) values > 1 in high molecular weight (HMW) n-alkanes in the hypersaline, metahaline, and euhaline regions of the Pulicat lagoon, India. However, in sub-surface sediments (up to 70 cm depth), the CPI values are consistently < 1 in the hypersaline region, but > 1 in the euhaline and metahaline regions. In the hypersaline region, the alteration of plant biomolecules possibly results from halophilic bacterial degradation and fatty acid to n-alkane reduction in anaerobic conditions (pristane/phytane values < 1), during the weaker northeast monsoon (44–60 cm sub-surface). Prolonged hypersaline conditions and associated anoxia resulted in a higher isotopic difference between fatty acids and n-alkanes (δ¹³C_FA-ALK value) in the hypersaline region (3.1 ± 1.5‰) compared to metahaline (2.2 ± 1.0‰) and euhaline (1.5 ± 1.0‰) regions. Although in the hypersaline region the HMW fatty acids have CPI values > 1, the higher δ¹³C_FA-ALK value also suggests the alteration of fatty acids. This study suggests that if plant-derived n-alkanes are altered, the δ¹³C values and distribution patterns of plant-derived fatty acids should be critically evaluated before using it as an alternative proxy in coastal environments.

在沉积物中，植物生物分子（正构烷烃和脂肪酸）在古环境研究中充当强有力的代表。尽管在土壤系统中控制植物生物分子保存和同位素组成的因素已被了解，但陆海界面沉积环境变化的影响，特别是在高盐环境中，并没有得到很好的限制。在这项研究中，我们观察到，在床沉积物中，植物生物分子以高分子量 (HMW) n的碳偏好指数 (CPI) 值 > 1 得以保存-印度普利卡特泻湖超盐、偏盐和真盐区域的烷烃。然而，在地下沉积物（深度达 70 厘米）中，高盐区域的 CPI 值始终 < 1，但在高盐和偏盐区域则 > 1。在高盐度地区，在较弱的东北季风期间（地下 44-60 厘米），植物生物分子的改变可能是由于厌氧条件下（降植烷/植烷值 < 1）嗜盐细菌降解和脂肪酸还原为正烷烃所致。。长期的高盐条件和相关的缺氧导致脂肪酸和正构烷烃之间较高的同位素差异（δ ¹³ C _FA-ALK值）在高盐区域（3.1±1.5‰）与偏盐（2.2±1.0‰）和高盐（1.5±1.0‰）区域相比。尽管在高盐区域，HMW 脂肪酸的 CPI 值 > 1，但较高的 δ ¹³ C _FA-ALK值也表明脂肪酸发生了变化。这项研究表明，如果植物来源的正构烷烃发生改变，则在将其用作沿海环境中的替代指标之前，应严格评估植物来源的脂肪酸的δ ^{13 C 值和分布模式。}