Phosphorus Speciation,Transformation,and Burial In Water-Sediment Systems In The Bohai And Yellow Seas

Phosphorus（P）is an essential nutrient for marine phytoplankton and is often considered to be the limiting element for primary productivity of the ocean on geological time scales,playing a critical role in regulating ecosystem function and biogeochemical cycling.Enhanced human activities have led to environmental problems such as offshore eutrophication,ocean acidification,and hypoxia,which have changed the concentration and form of phosphorus present in offshore ecosystems,as well as the original biogeochemical cycling process of phosphorus.Endogenous phosphorus transformations and exchanges between water bodies and sediments in nearshore waters are an important source of phosphorus in the water column.However,research on key processes of phosphorus cycling in the offshore water-sediment system is still limited,especially the sedimentary phosphorus cycling processes and their driving mechanisms as well as the coupling processes with the water column need to be further clarified.This is crucial for controlling the endogenous pollution of marine phosphorus and solving the related offshore environmental problems.In this study,the composition,distribution and transport transformation of phosphorus in Bohai Sea water and sediment were studied based on the analysis of different forms of phosphorus in water bodies,surface sediments and sediment column samples obtained from typical habitat areas and key sections in the Bohai Sea,and the burial and release mechanisms of sedimentary phosphorus were explored.Combining phosphate oxygen isotopes and hydrogen oxygen isotopes to reveal the transport of terrestrial phosphorus and the transformation of endogenous phosphorus in the Bohai sea and Yellow Seas,and to analyze the key processes of phosphorus cycle in the water-sediment system.On a long time scale,the sedimentary environment and long time scale phosphorus burial and recirculation mechanisms in the central muddy zone of the South Yellow Sea（core area of the Yellow Sea Cold Water Mass）were investigated in conjunction with foraminiferal dating,XRF core scanning and phosphorus fugitive morphology,and the long-term changes and indicative significance of phosphorus were explored.In summary,the combination of modern processes and sedimentary records,short time scales and long time scales was achieved,and key processes in the phosphorus cycle of the Bohai Yellow seawater-sediment system were analyzed from a new perspective.Major results in the thesis are summarized as follows:（1）The range of dissolved inorganic phosphorus（DIP）concentrations in the water column was 0.04～3.45μmol/L in summer and 0.17～0.93μmol/L in spring.The phosphorus cycle in surface waters adjacent to the Yellow River and Yangtze River estuaries was mainly influenced by freshwater inputs.The vigorous photosynthesis of phytoplankton in summer and the invasion of Yellow Sea warm current in winter resulted in lower DIP concentration in surface water in most areas of Bohai Sea in summer than that in spring.The seasonal generation and dissipation of Yellow Sea cold water mass was the main factor affecting the phosphorus cycle in the deep water body in the middle of the central Yellow Sea,and the limitation of vertical mixing in summer led to the high DIP concentration in this region.（2）Based on the analysis of phosphorus morphology in the surface sediments of key sections of the Bohai and Yellow Seas in summer,it was found that the preservation and release of phosphorus in the sediments are very sensitive to the physical processes and redox conditions of the surrounding water column.The spatial and temporal distribution of total phosphorus（Tot-P）content in the surface sediments of the Bohai and Yellow Seas varied widely,and inorganic phosphorus（IP）was the main content（84.4%）.Detrial phosphorus（Detr-P）had the highest mean content（8.89±2.35μmol/g）among the five fugitive forms of phosphorus,with high values occurring near the mouth of the Yellow River and the Yangtze River,where it was significantly influenced by land-based inputs.The seasonal hypoxia（minimum value of 0.39 mg/L）in the waters adjacent to the Yangtze River estuary contributed to the release of DIP from sediments to the overlying water.The resuspension of sediments in the area affected by upwelling resulted in the release of exchangeable phosphorus（Exch-P）into the water or the conversion to Fe-bound phosphorus（Fe-P）,with the lowest Exch-P value of 0.06μmol/L.The highest BAP content（8.59μmol/g）was found in the surface sediments of the waters affected by the Yellow Sea cold water mass,which had a high potential for DIP release to the water column and further promoted authigenic phosphorus（Auth-P）production.The ratios of total organic carbon to organic phosphorus（TOC/Org-P）in the seas adjacent to the estuary was attributed to the high TOC input from terrestrial sources and the increased primary productivity due to high nutrients,while the presence of the Yellow Sea cold water mass made the preservation of TOC and Org-P more efficient.（3）The phosphate oxygen isotope(δ¹⁸O_P)of the water column the Bohai and Yellow Seas ranged from 13.38‰to 23.87‰,and the equilibrium fractionation rate of phosphate oxygen isotopes was strongly influenced by temperature.Theδ¹⁸O_P of the water column were lower than the equilibrium value in the deep-water column influenced by the Yellow Sea cold water mass due to the lower temperature and the limitation of vertical mixing resulting in weaker biological activities,and the DIP in the water column is not fully recycled by organisms.Seasonal hypoxia in the deep-water column at stations adjacent to the Yangtze estuary led to the release of endogenous phosphorus from the sediment,and the combined resuspension of upwelling led to higher than equilibriumδ¹⁸O_P in the-deep water column in this region.The surface water near estuaries was affected by phosphorus limitation,and dissolved organic phosphorus（DOP）may be mineralized and decomposed as an alternative phosphorus source with the participation of extracellular enzymes,and kinetic fractionation led to lower than equilibrium values ofδ¹⁸O_P in regenerated phosphate in the ocean.The short turnover time of the water column under the influence of freshwater input from terrestrial sources may lead to the failure of oxygen isotope fractionation balance.In contrast,the proportion of DOP mineralized for reuse in the rest of the sea may be low.In addition,most of the stations in the study area were in N-limited or Si-limited states,which inhibited phytoplankton photosynthesis,and the biocirculation involved in DIP was not strong,so theδ¹⁸O_Pvalue deviated from the equilibrium state.（4）The evolution of the depositional environment and the burial characteristics of sedimentary phosphorus since～12.5 cal Ka BP were investigated based on the dating analysis of sediment cores from the central muddy zone of the South Yellow Sea.The sediment content of Tot-P,BAP and IP ranged from 11.3 to 16.3μmol/g,1.96 to 7.40μmol/g and 7.77 to 13.1μmol/g,respectively.On the whole,the contents of Exch-P and Fe-P in the column sediments showed similar trends with depth,ranging from 0.14 to0.82μmol/g and 1.45 to 1.69μmol/g,respectively,reflecting the predominance of Fe-P in the conversion of reactive phosphorus（Reac-P）during early sedimentation and diagenesis processes.It also showed that Org-P can reflect the change of primary productivity to some extent.The phosphorus burial characteristics and elemental changes in the core showed some response to climate change and environmental evolution.1)About 12.5～11.5 cal Ka BP was in the Younger Dryas Event period,phosphorus burial showed the characteristics of phosphorus cycle was influenced by hypoxic conditions,the content of Detr-P was dominant in Tot-P content,and continental deposition may dominate phosphorus burial at this stage,which was consistent with the high value indicating terrigenous input（Ti）in XRF.2)During the period from 11.5 to 8.2 cal Ka BP,the transition from nearshore to shallow marine sedimentation took place,with a gradual increase in East Asian Summer Monsoon（EASM）intensity,rapid sea level rise,and sedimentation processes influenced by river and tidal interactions,resulting in distinct changes in the elemental properties and phosphorus cycle of the core from the previous stage.The Detr-P content（7.36±0.39μmol/g）in this stage was the highest among all stages,while the Org-P content（1.46±0.2μmol/g）was the lowest.Auth-P was the main buried component of Reac-P,accounting for about 40%～65%of Reac-P.3)Approximately 8.2～4.6 cal Ka BP was in the shallow sedimentary phase,and the Org-P and Tot-P contents increased with the formation of the modern circulation pattern in the Yellow Sea as sea level rose.The formation of the circulation and the seasonal water mass production and elimination due to the long-term existence of the circulation may be the reason for the increase of Auth-P and Org-P abundances in core after the formation of the modern circulation pattern in the South Yellow Sea.With the rise of sea level and the formation of estuarine deltas,the decrease of TOC/TN（Total nitrogen）indicates that the proportion of organic matter caused by marine organisms was gradually increasing.4)The EASM intensity was low from about 4.6 cal Ka BP to the present,when sediments were more influenced by enhanced human activities and the mean reactive phosphorus（Reac-P）content（9.49±0.61μmol/g）in sediments reached the highest of all phases,and organic matter from primary production of marine phytoplankton could dominate phosphorus burial in the shallow sedimentary environment.On long time scales,sedimentary phosphorus cycle characteristics responded to periods of significant anthropogenic influence and periods of abrupt climate change,and the marine phosphorus cycle influenced marine primary productivity.