## 3 Experimental Results
### EDXRF Analysis Results of the Attached Marine Organism Remains  
Table 1 lists the energy-dispersive X-ray fluorescence (EDXRF) analysis of various types of marine organism remains. The results showed that the Ca contents in the remains of several organisms were higher than 70 wt%, originating from the calcareous skeletons formed after biomineralization. Among all known biominerals, approximately half are calciferous, highlighting the key functional role of Ca in cellular metabolism. Calcium carbonate is the most abundant biomineral [19]. Comparing the chemical compositions of several types of organism remains, it was found that the Mg contents in red coral and Serpulid remains were higher than those in Bryozoan and solitary coral remains. This might be attributed to different crystal phases of calcium carbonates in organic skeletons and varying substitution degrees of Mg in calcium carbonate lattices, further discussed in  
Figure 2: OM images of the marine organism remains attached to sample DL: (**a**) Solitary organism: solitary coral remains on the outer wall of sample DL-4; (**b**) Modular organism: Serpulid remains on the outer wall of sample DL-2; (**c**) Red coral remains with light red color on the outer wall of sample DL-2; and (**d**) Bryozoan remains on the inner wall of sample DL-5. Section 3.4. The Bryozoans and tube worms attached to the surface of the samples might have been contaminated by marine sediments such as quartz and feldspar, resulting in higher Al and Si contents. Additionally, 3-4 wt% of Fe in the chemical composition of several marine organism remains, except for solitary coral, might come from contamination by iron rust [13]. The element S was found in several types of organism remains, primarily sourced from two aspects. During the biomineralization process, organisms require highly negatively charged sulfated organic macromolecules to attract and bond bivalent cations such as Ca\({}^{2+}\) and Mg\({}^{2+}\). [PERSON] et al. also found sulfated polysaccharides in the lining of Serpulids [20; 21]. Another source is sulfate in the sea or sulfides in other forms. The _Desulfovibrio_ spp. in the seafloor sediments are active, and the sulfate might be reduced to sulfide or exist in other low-valency sulfur forms [2; 22]. The elements Na and Cl, as two major elements in seawater, except H and O, were slightly detected in several types of organism remains. Figure 3 displays the element mapping images of an area on the inner wall of DL with a significant amount of organism remains; Table 2 lists the chemical composition analysis results. As shown in Figure 3c,d, elements Al and Si are mainly concentrated in the top left, where fewer foreign pollutants were found, and the body is partly exposed. The chemical composition at position P1 also revealed high contents of Si and Al in the body. The Fe content was also as high as 15 wt%, suggesting the deposition of foreign pollutants with rusts. Figure 3e shows the element distribution in the top right and lower part where red corals were attached, indicating a high concentration of Ca. According to the chemical compositions at positions P2, P5, and P7, the Ca content was the highest (95 wt%). This indicated that the marine organism remains attached to the inner wall of DL were calcareous. The dark coagulum in the selected area was mainly distributed in the middle part. The element mapping images demonstrated the enrichment of Al, Si, and Fe (Figure 3c,d,f). The chemical composition at positions P3 and P4 suggested that the Fe and Si contents in the dark coagula area were 35-47 wt% and 21-31 wt%, respectively. This could be attributed to the deposition of foreign rusts, sea sand, and sea mud. In addition, the P, S, and Cl contents at positions P5 and P6 were obviously higher than those at other positions.